I worked on this quite a bit more tonight, and eventually found that it is probably some manner of obscure nameserver problem, as replacing mail.sonic.net with the EMSTP server's IP address got outgoing email working. But I'm at a loss as to what the NS problem is, if so. I can do DNS lookups, dig, telnet to port 587, all that stuff - yet Postfix insists on coughing up a "relayhost configuration problem" / "unable to look up host mail.sonic.net: Name or service not known" errors when trying to relay mail. N.b. I have to use Google nameservers, as the Sonic ones don't seem to allow lookups from a Comcast IP (I am just an email customer until Sonic rolls fiber in Hayward
I haven't changed anything relevant in my OS or postfix setup in the timeframe that this problem started happening, so it seems likely to be *something* happening on the Sonic end.