SMTP provides for:
address-literal = "[" ( IPv4-address-literal /
General-address-literal ) "]"
; See Section 4.1.3
Mailbox = Local-part "@" ( Domain / address-literal )
However, it is not clear how mailboxes containing an address literal are to be
treated by an MTA when they appear in the envelope. Specifically, I have three
questions, assuming a recipient is indicated using an address literal:
1. If the host can resolve the address and obtain a domain name, it can replace
it so as to obtain a regular mailbox. Right?
2. If the host cannot resolve the address, it can either try to relay it or try
to deliver it locally. What is the recommended behavior?
3. If in case (2) the server relays, it can either drop or keep the domain part
when it issues RCPT TO to its peer. Dropping is conservative, as the address
literal has already be used, so it won't be used again in case the peer is the
server itself connected through a NAT or transparent proxy. OTOH, dropping
precludes another rDNS attempt, which could work at the peer if its internal
DNS view provided for an rDNS delegation which was not granted globally. To
drop or not to drop?
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