If I grep 587 /etc/services I find:
submission 587/tcp # Submission [RFC2476]
Upon reading
http://www.ietf.org/rfc/rfc2476.txt, albeit not
exhaustively, it seems the idea is that port 587 should be used to
submit an email by an email client (MUA, or MSA, the abbreviations
rock). And port 25 is used by MTAs to transfer emails.
I wonder how this happens in practice. It seems to me port 25 is widely
used for anything smtp, be it sending, receiving, smtp_auth, encryption.
And then there is port 465 (ssmtp), which is also still widely used, and
to quote a debian exim4 readme (/usr/share/doc/exim4-base/README.Debian.gz):
"Some broken clients (most prominent example being nearly all versions
of Microsoft Outlook and Outlook Express) insist on doing TLS on connect
on Port 465."
To quote the rfc:
"While most email clients and servers can be configured to use port 587
instead of 25, there are cases where this is not possible or convenient.
A site MAY choose to use port 25 for message submission, by
designating some hosts to be MSAs and others to be MTAs."
One may argue why bother with 587 at all if you are still allowed to use 25.
What is wisdom in this mess? Insist on everyone using port 587? ignore
587 and support a host of legacy clients with port 465 and allow port 25
for submissions? Open all 3 ports and allow whatever people want on any
port? Users have been told for years to use 25 (and 465), to add to the
confusion.
Out of curiosity, why is it so wrong to use 465? It's just a port
number, not a religion. :-) Instead of choosing another port, those who
"decide" (who?) could have renamed port 465 to read "submission". Or not?
Thank you,
Jeroen
