I wonder how long before ISPs start blocking 587. Certainly it doesn't take much for spammers to switch to 587 as well.
There is a reason why an RFC defined port 587 when port 25 already existed. :)
Port 25 is used for any SMTP connections -- from MUA (mail client) to MTA (mail server) and MTA to MTA. Many ISPs now block outgoing connection to port 25 since their customers should not be sending out E-mail directly to other mailservers (as that is how spammers operate), and it is impossible to distinguish MUA->MTA traffic from MTA->MTA traffic.
However, port 587 traffic is *only* used for MTA->MTA traffic. For example, if @example.com has an MX record pointing to a host that resovles to 192.0.2.25, that IP is required to accept mail on port 25. If it also accepts E-mail on port 587, it should only accept E-mail from authenticated users. If it accepts E-mail from non-authenticated users (I haven't heard of it happening yet), it will almost certainly get blacklisted. So ISPs have no need to block port 587 traffic.
-Scott
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