> Why not make it be qmtp.tcp.domain, and use the port as the port to > connect to for qmtp? First, because the standardized form of the SRV lookup will have underscores in front of the service name and protocol name. So it would be _qmtp._tcp.domain. Second, because that wouldn't qualify as "using SRV as a protocol switch," at least not in an efficient way. If there are N different protocols for transferring mail then you'd have to do N different DNS lookups. If you have a generic mail lookup then you can use the well-known port numbers to switch between N different protocols with only one lookup. Of course, by using well-known ports to identify protocols you lose the flexibility of running a QMTP or SMTP server on a non-well-known port and being able to point people at it. But MX never gave you that flexibility anyway, so I don't expect people to consider it important. This is, of course, all academic in the current context. If we were an IETF working group working on mail protocols, we could debate whether to have a single SRV record or one SRV record for each protocol and it would actually matter (if we had decided to use SRV lookups for mail transfer at all). But this is just the qmail list and Dan hasn't stated any intention to consider using SRV lookups in qmail.
