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From: Andrew Gideon on 23 Apr 2008 14:47
On Wed, 23 Apr 2008 12:40:10 -0400, John Murtari wrote:

> I work at a web host, we tell people to change to 587 in cases where
> their ISP blocks port 25 connections attempting to leave their network,
> i.e. they are trying to stop spammers.

I don't know the history, but somewhere along the way 587 became the
standard "email submission" port. The idea is that this is distinct from
"email transfer", the latter being server-server and the former being
client-server.

This permits different rules (or even different softwares!) to be applied
to the different ports. Incoming traffic on port 25 must be to a domain
handled by that server, for example, while incoming traffic on port 587
must be authenticated.

At least some SMTP servers (ie. sendmail) can be configured to handle
"local domain or authenticated" on a single port, which is where I suffer
from a bit of ignorance: I don't know why the desire for different ports
arose. Perhaps to run different softwares for the different audiences w/
o wasting IPv4 space?

- Andrew
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