diff --git a/backend.go b/backend.go
index d61f3f6..c2909da 100644
--- a/backend.go
+++ b/backend.go
@@ -42,7 +42,7 @@ type MailOptions struct {
 	Body BodyType
 
 	// Size of the body. Can be 0 if not specified by client.
-	Size int
+	Size int64
 
 	// TLS is required for the message transmission.
 	//
diff --git a/client.go b/client.go
index 66ff78f..7a99a10 100644
--- a/client.go
+++ b/client.go
@@ -384,7 +384,7 @@ func (c *Client) Mail(from string, opts *MailOptions) error {
 		cmdStr += " BODY=8BITMIME"
 	}
 	if _, ok := c.ext["SIZE"]; ok && opts != nil && opts.Size != 0 {
-		cmdStr += " SIZE=" + strconv.Itoa(opts.Size)
+		cmdStr += fmt.Sprintf(" SIZE=%v", opts.Size)
 	}
 	if opts != nil && opts.RequireTLS {
 		if _, ok := c.ext["REQUIRETLS"]; ok {
diff --git a/conn.go b/conn.go
index 46662e1..0225876 100644
--- a/conn.go
+++ b/conn.go
@@ -37,7 +37,7 @@ type Conn struct {
 	bdatPipe        *io.PipeWriter
 	bdatStatus      *statusCollector // used for BDAT on LMTP
 	dataResult      chan error
-	bytesReceived   int // counts total size of chunks when BDAT is used
+	bytesReceived   int64 // counts total size of chunks when BDAT is used
 
 	fromReceived bool
 	recipients   []string
@@ -321,12 +321,12 @@ func (c *Conn) handleMail(arg string) {
 				return
 			}
 
-			if c.server.MaxMessageBytes > 0 && int(size) > c.server.MaxMessageBytes {
+			if c.server.MaxMessageBytes > 0 && size > c.server.MaxMessageBytes {
 				c.writeResponse(552, EnhancedCode{5, 3, 4}, "Max message size exceeded")
 				return
 			}
 
-			opts.Size = int(size)
+			opts.Size = size
 		case "SMTPUTF8":
 			if !c.server.EnableSMTPUTF8 {
 				c.writeResponse(504, EnhancedCode{5, 5, 4}, "SMTPUTF8 is not implemented")
@@ -676,7 +676,7 @@ func (c *Conn) handleBdat(arg string) {
 		return
 	}
 
-	if c.server.MaxMessageBytes != 0 && c.bytesReceived+int(size) > c.server.MaxMessageBytes {
+	if c.server.MaxMessageBytes != 0 && c.bytesReceived+int64(size) > c.server.MaxMessageBytes {
 		c.writeResponse(552, EnhancedCode{5, 3, 4}, "Max message size exceeded")
 
 		// Discard chunk itself without passing it to backend.
@@ -746,7 +746,7 @@ func (c *Conn) handleBdat(arg string) {
 		return
 	}
 
-	c.bytesReceived += int(size)
+	c.bytesReceived += int64(size)
 
 	if last {
 		c.lineLimitReader.LineLimit = c.server.MaxLineLength
diff --git a/server.go b/server.go
index 0e63cc7..a278fe2 100644
--- a/server.go
+++ b/server.go
@@ -40,7 +40,7 @@ type Server struct {
 
 	Domain            string
 	MaxRecipients     int
-	MaxMessageBytes   int
+	MaxMessageBytes   int64
 	MaxLineLength     int
 	AllowInsecureAuth bool
 	Debug             io.Writer