Quoting "Stuart D. Gathman" <stuart(_at_)bmsi(_dot_)com>:
SMTP submission port is arbitrary. If the "connectivity provider" blocks
*all* outgoing TCP ports, what exactly are you paying them for?
Hint: I'm not.
Pick a port that they *don't* block, and have your users do SMTP AUTH.
My daytime service provider blocks all ports except 80 and 443. And your next
comment is pretty unrealistic.
Shoot, you can do SMTP AUTH over port 80 if required.
Yeah, that'll happen.
BTW, blocking outgoing tcp 25 is common, but I've never heard of an ISP
blocking 587. What would be the point?
Guaranteeing that every email packet goes through your own servers, of course.
So you can take responsibility for the content.
I could imagine a service that provides access to web proxies, imap servers,
and smtp servers, but doesn't actually give you any access to the internet.
Calling such a service a "connectivity provider" is an exercise in
Orwellian speech.
Actually, for other ISPs they're the dream partner: the sending ISP that really
can take responsibility for every packet they send, without unproductive
restrictions such as SPF.