Detect and break forwarding loops

[foxcpp]

My initial idea is to inspect Received fields in the headers of the messages.
If it contains value mentioning the recipient address we are handling and our hostname - we bounce the message because it was processed by our server already.

It is important to check hostname because a single recipient may be processed by multiple servers and each will include its own Received field.
Another consideration to take into account: We should skip Received fields we can't process as some MTAs include information using non-standard formats (qmail, for example). Actually, SMTP RFC requires MTA to not fail delivery if an invalid Received field is present in the header.

Postfix relies on Delivered-To (non-standard AFAIK) fields for this purpose. We might want to include this field when the message leaves the pipeline. Currently, it is included only when the message is stored in an IMAP mailbox.

[foxcpp]

Alternatively, we may want to reject messages that contain a lot of Received fields in header.
Borrowing default limits from Postfix: It rejects messages if they contain more than 50 Received fields.

[foxcpp]

https://tools.ietf.org/html/rfc5321#section-6

Whatever mechanisms are used, servers MUST
contain provisions for detecting and stopping trivial loops.

Interestingly, it is actually a "MUST" requirement in RFC 5321.