POP3 support

[kiwiz]

Use case

Interfacing with clients that only support POP3 (Gmail, for example)

Your idea for a solution

Adding POP3 support to maddy

• I'm willing to help with the implementation

[foxcpp]

I would simply map POP3 operations to corresponding IMAP ones and reuse same storage interfaces and so on. We need to have a reasonably robust library to decode/encode wire protocol, the rest should be fairly easy.

PRs are welcome, let me know if you need any guidance on where to start.

[foxcpp]

https://github.com/regnull/popgun might be a good starting point.

[kiwiz]

A couple of questions/comments:

• Implementing this is as easy* as creating internal/endpoint/pop3/pop3.go and importing the package in maddy.go?
• Is there a preference for integrating the logic into maddy vs just depending on regnull/popgun?
  • My guess is the former as popgun hardcodes the default logger and doesn't let you specify multiple listeners

[foxcpp]

Implementing this is as easy* as creating internal/endpoint/pop3/pop3.go and importing the package in maddy.go

Would need to figure how to map IMAP-specific storage interface to POP3 operations. Might be:

• Lock -> Select mailbox
• Unlock -> Close mailbox
• Update -> Expunge
• Rset -> ???
• Dele -> Store +FLAGS \Deleted
• List, Retr, Top -> Fetch

Lock/Unlock would also need to create per-user lock somehow, not exactly sure how that would work.

Is there a preference for integrating the logic into maddy vs just depending on regnull/popgun?

My guess is the former as popgun hardcodes the default logger and doesn't let you specify multiple listeners

I would fork popgun and do necessary adjustments to support pluggable logger and multiple listeners. And then use that as a library in main codebase.