diff --git a/responders/MSDefenderOffice365/Dockerfile b/responders/MSDefenderOffice365/Dockerfile
new file mode 100644
index 000000000..7a1aacb69
--- /dev/null
+++ b/responders/MSDefenderOffice365/Dockerfile
@@ -0,0 +1,23 @@
+FROM python:3.10-slim
+
+# Install system components
+RUN apt-get update  && apt-get install -y curl gnupg apt-transport-https
+
+# Import the public repository GPG keys
+RUN curl -sS https://packages.microsoft.com/keys/microsoft.asc | apt-key add -
+
+# Register the Microsoft Product feed
+RUN sh -c 'echo "deb [arch=amd64] https://packages.microsoft.com/repos/microsoft-debian-bullseye-prod bullseye main" > /etc/apt/sources.list.d/microsoft.list'
+
+# Install PowerShell (/usr/bin/pwsh)
+RUN apt-get update && apt-get install -y powershell
+
+COPY install_deps.ps1 .
+RUN pwsh -File install_deps.ps1
+
+WORKDIR /worker
+COPY requirements.txt .
+RUN pip3 install --no-cache-dir -r requirements.txt
+COPY . .
+
+ENTRYPOINT ["python3", "ms_defender_office.py"]
\ No newline at end of file
diff --git a/responders/MSDefenderOffice365/MSDefenderOffice365_block.json b/responders/MSDefenderOffice365/MSDefenderOffice365_block.json
new file mode 100644
index 000000000..640649168
--- /dev/null
+++ b/responders/MSDefenderOffice365/MSDefenderOffice365_block.json
@@ -0,0 +1,68 @@
+{
+    "name": "MSDefenderOffice365_block",
+    "version": "1.0",
+    "author": "Joe Lazaro",
+    "url": "https://github.com/TheHive-Project/Cortex-Analyzers",
+    "license": "AGPL-V3",
+    "description": "Add entries to the Tenant Allow/Block List in the Microsoft 365 Defender",
+    "dataTypeList": [
+        "thehive:case_artifact"
+    ],
+    "command": "MSDefenderOffice365/ms_defender_office.py",
+    "baseConfig": "MSDefenderOffice365",
+    "config": {
+        "service": "block"
+    },
+    "registration_required": true,
+    "subscription_required": true,
+    "free_subscription": false,
+    "service_homepage": "https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/defender-for-office-365?view=o365-worldwide",
+    "service_logo": {
+        "path": "assets/MicrosoftDefenderForOffice365_logo.png",
+        "caption": "logo"
+    },
+    "screenshots": [
+        {
+            "path": "assets/MSDefenderOffice365_Block.png",
+            "caption": "Example responder action result"
+        }
+    ],
+    "configurationItems": [
+        {
+            "name": "certificate_base64",
+            "description": "Base64-encoded PFX certificate to be used for certificate-based authentication.",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "certificate_password",
+            "description": "Password for the certificate used to authenticate",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "app_id",
+            "description": "The application ID of the service principal that's used in certificate based authentication",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "organization",
+            "description": "Tenant ID. Example: something.onmicrosoft.com",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "block_expiration_days",
+            "description": "How many days out should we set the expiration? A value <= 0 means to set no expiration.",
+            "type": "number",
+            "multi": false,
+            "required": true,
+            "defaultValue": 0
+        }
+    ]
+}
\ No newline at end of file
diff --git a/responders/MSDefenderOffice365/MSDefenderOffice365_unblock.json b/responders/MSDefenderOffice365/MSDefenderOffice365_unblock.json
new file mode 100644
index 000000000..7c0a72c14
--- /dev/null
+++ b/responders/MSDefenderOffice365/MSDefenderOffice365_unblock.json
@@ -0,0 +1,60 @@
+{
+    "name": "MSDefenderOffice365_unblock",
+    "version": "1.0",
+    "author": "Joe Lazaro",
+    "url": "https://github.com/TheHive-Project/Cortex-Analyzers",
+    "license": "AGPL-V3",
+    "description": "Add entries to the Tenant Allow/Block List in the Microsoft 365 Defender",
+    "dataTypeList": [
+        "thehive:case_artifact"
+    ],
+    "command": "MSDefenderOffice365/ms_defender_office.py",
+    "baseConfig": "MSDefenderOffice365",
+    "config": {
+        "service": "unblock"
+    },
+    "registration_required": true,
+    "subscription_required": true,
+    "free_subscription": false,
+    "service_homepage": "https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/defender-for-office-365?view=o365-worldwide",
+    "service_logo": {
+        "path": "assets/MicrosoftDefenderForOffice365_logo.png",
+        "caption": "logo"
+    },
+    "screenshots": [
+        {
+            "path": "assets/MSDefenderOffice365_Block.png",
+            "caption": "Example responder action result"
+        }
+    ],
+    "configurationItems": [
+        {
+            "name": "certificate_base64",
+            "description": "Base64-encoded PFX certificate to be used for certificate-based authentication.",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "certificate_password",
+            "description": "Password for the certificate used to authenticate",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "app_id",
+            "description": "The application ID of the service principal that's used in certificate based authentication",
+            "type": "string",
+            "multi": false,
+            "required": true
+        },
+        {
+            "name": "organization",
+            "description": "Tenant ID. Example: something.onmicrosoft.com",
+            "type": "string",
+            "multi": false,
+            "required": true
+        }
+    ]
+}
\ No newline at end of file
diff --git a/responders/MSDefenderOffice365/README.md b/responders/MSDefenderOffice365/README.md
new file mode 100644
index 000000000..8ce86c610
--- /dev/null
+++ b/responders/MSDefenderOffice365/README.md
@@ -0,0 +1,14 @@
+Microsoft Defender for Office 365 safeguards your organization against malicious threats posed by email messages, links (URLs), and collaboration tools. Defender for Office 365 includes:
+
+* Threat protection policies: Define threat-protection policies to set the appropriate level of protection for your organization.
+* Reports: View real-time reports to monitor Defender for Office 365 performance in your organization.
+* Threat investigation and response capabilities: Use leading-edge tools to investigate, understand, simulate, and prevent threats.
+* Automated investigation and response capabilities: Save time and effort investigating and mitigating threats.
+
+This responder implements support for the Tenant Allow/Block List which is used during mail flow for incoming messages to manually override the Microsoft 365 filtering verdicts. An observable with dataType 'mail' is used to block/unblock a sender, while dataType 'domain' is used to block/unblock a domain.
+
+You can also block or unblock multiple entries at once by using a multi-line observable with one entry per line.
+
+The configuration allows you to specify the number of days for a block entry to live before expiration with a value of 0 meaning no expiration.
+
+For further reference on this capability, see the Microsoft documentation [Allow or block emails using the Tenant Allow/Block List](https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/allow-block-email-spoof?view=o365-worldwide).
diff --git a/responders/MSDefenderOffice365/assets/MSDefenderOffice365_Block.png b/responders/MSDefenderOffice365/assets/MSDefenderOffice365_Block.png
new file mode 100644
index 000000000..efb82f43b
Binary files /dev/null and b/responders/MSDefenderOffice365/assets/MSDefenderOffice365_Block.png differ
diff --git a/responders/MSDefenderOffice365/assets/MicrosoftDefenderForOffice365_logo.png b/responders/MSDefenderOffice365/assets/MicrosoftDefenderForOffice365_logo.png
new file mode 100644
index 000000000..4472a196f
Binary files /dev/null and b/responders/MSDefenderOffice365/assets/MicrosoftDefenderForOffice365_logo.png differ
diff --git a/responders/MSDefenderOffice365/install_deps.ps1 b/responders/MSDefenderOffice365/install_deps.ps1
new file mode 100644
index 000000000..ebf5b4930
--- /dev/null
+++ b/responders/MSDefenderOffice365/install_deps.ps1
@@ -0,0 +1,4 @@
+Set-PSRepository -Name "PSGallery" -InstallationPolicy Trusted
+Install-Module -Name ExchangeOnlineManagement
+Install-Module -Name PSWSMan -Scope AllUsers -RequiredVersion 2.3.0
+Install-WSMan
diff --git a/responders/MSDefenderOffice365/ms_defender_office.py b/responders/MSDefenderOffice365/ms_defender_office.py
new file mode 100755
index 000000000..f3498b341
--- /dev/null
+++ b/responders/MSDefenderOffice365/ms_defender_office.py
@@ -0,0 +1,154 @@
+#!/usr/bin/env python3
+import json
+import os
+import re
+import subprocess
+import tempfile
+from base64 import b64decode
+from pathlib import Path
+
+from cortexutils.responder import Responder
+
+
+class MsDefenderOffice365Responder(Responder):
+    def __init__(self):
+        Responder.__init__(self)
+        self.service = self.get_param(
+            'config.service', None, 'Service parameter is missing')
+        self.certificate_base64 = self.get_param(
+            'config.certificate_base64', None,
+            'Config is missing certificate_base64')
+        self.certificate_password = self.get_param(
+            'config.certificate_password', None,
+            'Config is missing certificate_password')
+        self.app_id = self.get_param(
+            'config.app_id', None,
+            'Config is missing app_id')
+        self.organization = self.get_param(
+            'config.organization', None,
+            'Config missing organization')
+        self.block_expiration_days = self.get_param(
+            'config.block_expiration_days', 0)
+        self.script_dir = os.path.join(Path(__file__).absolute(), 'scripts')
+
+    def clean_output(self, stream: bytes):
+        """Decode byte stream and remove ANSI color codes"""
+        re_no_ansi = re.compile(r'\x1b[^m]*m')
+        return re_no_ansi.sub('', stream.decode('utf-8'))
+
+    def run(self):
+        observable = self.get_data()
+        o_data = observable['data']
+        if isinstance(o_data, str) and '\n' in o_data:
+            o_data = o_data.splitlines()
+        if not isinstance(o_data, list):
+            o_data = [o_data]
+
+        if observable['dataType'] not in ['domain', 'fqdn', 'mail']:
+            self.error(f"Data type {observable['dataType']} not supported.")
+
+        try:
+            clean_cert64 = re.sub(r'\s+', '', self.certificate_base64)
+            cert_bytes = b64decode(clean_cert64)
+            temp_cert_file = tempfile.NamedTemporaryFile(suffix='.pfx')
+            temp_cert_file.write(cert_bytes)
+            temp_cert_file.flush()
+        except ValueError as e:
+            self.error(f"While loading the certificate data: {e}")
+
+        script_name = f'scripts/{self.service}_sender.ps1'
+        process_args = [
+            "pwsh",
+            script_name,
+            temp_cert_file.name,
+            self.certificate_password,
+            self.app_id,
+            self.organization,
+        ]
+        if self.service == 'block':
+            caseId = observable['case']['caseId']
+            process_args.append(f"TheHive case #{caseId}")
+            process_args.append(str(self.block_expiration_days))
+        process_args += o_data
+
+        try:
+            result = subprocess.run(
+                process_args,
+                capture_output=True)
+        except subprocess.TimeoutExpired:
+            self.error(f"Timeout waiting for {script_name} to complete."
+                       f"\nstdout: {self.clean_output(result.stdout)}"
+                       f"\nstderr: {self.clean_output(result.stderr)}")
+
+        scriptErr = self.clean_output(result.stderr)
+        if result.returncode != 0:
+            self.error(
+                f"The powershell script reported an error: {scriptErr}"
+                "\n\nThe non-error output was the following: " +
+                self.clean_output(result.stdout)
+            )
+
+        try:
+            # We should get back an array of dictionaries, one for each
+            # endpoint that was submitted for action.
+            scriptResult = self.clean_output(result.stdout)
+            re_json_list = re.compile(r'\[\s*\{.*\}\s*\]', re.DOTALL)
+            re_json_object = re.compile(r'\{.*\}', re.DOTALL)
+            match_json_list = re_json_list.search(scriptResult)
+            match_json_object = re_json_object.search(scriptResult)
+
+            if match_json_list is not None:
+                extractedJson = match_json_list.group()
+                scriptResultDict = json.loads(extractedJson)
+            elif match_json_object is not None:
+                extractedJson = match_json_object.group()
+                scriptResultDict = [json.loads(extractedJson)]
+            else:
+                self.error(
+                    "Failed to identify JSON in script output:" +
+                    scriptResult)
+        except json.JSONDecodeError as e:
+            self.error(f"Error decoding JSON: {e}"
+                       f"| Input: {extractedJson}")
+
+        successful_entries = []
+        error_entries = []
+        for item in scriptResultDict:
+            if item.get('error') is not None:
+                # Don't treat it as an error if the entry we're trying to
+                # unblock already exists
+                if 'Entry not found' in item['error']:
+                    successful_entries.append(
+                        f"{item['entry']}: Entry not found."
+                    )
+                else:
+                    error_entries.append(
+                        f"{item['entry']}: {item['error']}"
+                    )
+            else:
+                success_dict = json.loads(item['result'])
+                if self.service == 'block':
+                    result_msg = (f"{item['entry']} Expiration " +
+                                  str(success_dict['ExpirationDate']))
+                else:
+                    result_msg = item['entry']
+
+                successful_entries.append(result_msg)
+
+        if len(error_entries) > 0:
+            report = {
+                'message': "At least one endpoint action had an error.",
+                'errored_entries': error_entries,
+                'successful_entries': successful_entries,
+            }
+            self.error(json.dumps(report))
+        else:
+            report = {
+                'message': "All endpoint actions completed with no error",
+                'entries': successful_entries,
+            }
+            self.report(report)
+
+
+if __name__ == '__main__':
+    MsDefenderOffice365Responder().run()
diff --git a/responders/MSDefenderOffice365/requirements.txt b/responders/MSDefenderOffice365/requirements.txt
new file mode 100644
index 000000000..8ad52a568
--- /dev/null
+++ b/responders/MSDefenderOffice365/requirements.txt
@@ -0,0 +1 @@
+cortexutils
diff --git a/responders/MSDefenderOffice365/scripts/block_sender.ps1 b/responders/MSDefenderOffice365/scripts/block_sender.ps1
new file mode 100644
index 000000000..f24935460
--- /dev/null
+++ b/responders/MSDefenderOffice365/scripts/block_sender.ps1
@@ -0,0 +1,47 @@
+param (
+    [parameter(mandatory=$true)]
+    [string] $certFilePath,
+    [parameter(mandatory=$true)]
+    [string] $certPassword,
+    [parameter(mandatory=$true)]
+    [string] $appId,
+    [parameter(mandatory=$true)]
+    [string] $organization,
+    [string] $notes="",
+    [parameter(mandatory=$true)]
+    [int] $expirationLength,
+    [parameter(mandatory=$true, valueFromRemainingArguments=$true)]
+    [string[]] $entries
+)
+
+$connectSplat = @{
+    CertificateFilePath = $certFilePath
+    CertificatePassword = $(ConvertTo-SecureString -String $certPassword -AsPlainText -Force)
+    AppId = $appId
+    Organization = $organization
+}
+
+Connect-ExchangeOnline @connectSplat
+
+$allResults = @()
+ForEach ($entry in $entries) {
+    if ($expirationLength -le 0) {
+        # No expiration
+        $result = New-TenantAllowBlockListItems -ListType Sender -Block -Notes $notes -Entries $entry -NoExpiration -ErrorAction Continue | ConvertTo-Json
+        $allResults += @{
+            entry = $entry;
+            result = $result;
+            error = If ($?) {$null} else {$Error[0].Exception.SerializedRemoteException.Message};
+        }
+    } else {
+        $expiry = (Get-Date).AddDays($expirationLength)
+        $result = New-TenantAllowBlockListItems -ListType Sender -Block -ExpirationDate $expiry -Notes $notes -Entries $entry -ErrorAction Continue | ConvertTo-Json
+        $allResults += @{
+            entry = $entry;
+            result = $result;
+            error = If ($?) {$null} else {$Error[0].Exception.SerializedRemoteException.Message};
+        }
+    }
+}
+
+$allResults | ConvertTo-Json -Depth 4
diff --git a/responders/MSDefenderOffice365/scripts/unblock_sender.ps1 b/responders/MSDefenderOffice365/scripts/unblock_sender.ps1
new file mode 100644
index 000000000..a0d6119ec
--- /dev/null
+++ b/responders/MSDefenderOffice365/scripts/unblock_sender.ps1
@@ -0,0 +1,33 @@
+param (
+    [parameter(mandatory=$true)]
+    [string] $certFilePath,
+    [parameter(mandatory=$true)]
+    [string] $certPassword,
+    [parameter(mandatory=$true)]
+    [string] $appId,
+    [parameter(mandatory=$true)]
+    [string] $organization,
+    [parameter(mandatory=$true, valueFromRemainingArguments=$true)]
+    [string[]] $entries
+)
+
+$connectSplat = @{
+    CertificateFilePath = $certFilePath
+    CertificatePassword = $(ConvertTo-SecureString -String $certPassword -AsPlainText -Force)
+    AppId = $appId
+    Organization = $organization
+}
+
+Connect-ExchangeOnline @connectSplat
+
+$allResults = @()
+ForEach ($entry in $entries) {
+    $result = Remove-TenantAllowBlockListItems -ListType Sender -Entries $entry | ConvertTo-Json
+    $allResults += @{
+        entry = $entry;
+        result = $result;
+        error = If ($?) {$null} else {$Error[0].Exception.SerializedRemoteException.Message};
+    }
+}
+
+$allResults | ConvertTo-Json -Depth 4