An implementation can certainly cache a message during simultaneous
receive/send and defer storage until it is clear that the delivery
won't complete?
Tony Finch on Wed, 2 Feb 2005:
I doubt any proper MTAs work in this way, but some firewalls implement
SMTP proxies (pejoratively known as "SMTP fux-up mode" because of the
gratuitous incompetence of many implementations). These are probably in
the target area for OPES.
> And how many try a direct forward and store only if that does not work out?
I don't think there are any. That would imply attempting to do a
complete delivery of the message to its next hop in the time between
CRLF.CRLF and its response, which is against the following text from RFCs
1123 and 2821:
To avoid receiving duplicate messages as the result of timeouts, a
receiver-SMTP MUST seek to minimize the time required to respond to
the final <CRLF>.<CRLF> end of data indicator. See RFC 1047 [28] for
a discussion of this problem.