Support IMAP4 Login Referrals

[lberezy]

I'm wondering if support for RFC 2221 (IMAP4 Login Referrals) is planned?

It's possible to slightly work around this now by checking the error message thrown upon login or authenticate and parsing the message manually, but it seems like this could be a useful integration.

The current real-world case that prompted this question is a dovecot cluster, where a user's home mailbox exists on another server.
This will result in either:

• No Response: [REFERRAL imap://test%40mail1.example.com;AUTH=*@mail2.example.com/] after authentication against the other cluster member.
• Or No Response: [REFERRAL imap://test%40mail1.example.com;AUTH=*@/] in the case where the authentication is handled by another cluster member, but the server is unavailable (e.g. due to maintenance).

[jonhoo]

Ah, interesting! It's not something I've looked at or planned, but it seems like a useful feature. I'd be happy to review a PR if you want to try to write something up? I don't think it should be too difficult :)