[Evolution] I don't understand evolution's use of ports

[Leonard Evens <le335160 gmail com>]

I was having enormous trouble configuring evolution on my Fedora 14
system to use smtp.gmail.com as an outgoing mail server.  I switched to
using comcast, and that worked fine.  But I then went back and checked
and found I had put
smtp.comcast.net
instead of
smtp.comcast.net:587
When I added the ':587', then the comcast server stopped working.
I then checked the gmail server and found I had added ':587'.  I then
deleted it, and the gmail server started working.

Let me explain.  Originally, everyone by default used port 25.  But
ISPs, including comcast, my ISP, started blocking outpu t using prot 25
because spammers were using it.  My outgoing email got blocked as a
result.To fix it, I had to configure the server I was using to accept
port 587 and then I had to set the server name to be
smtp:xxx,xxx,xxx:587
and that is still the way my evolution account for that server is set
up.  And it still works.

Now I think that comcast's server will not accept port 25, so it must be
using 587.  Similarly for the gmail server.  So why does adding that
':587' stop outgoing mail from working?

I find this very mysterious.

Can anyone explain what is going on, and/or refer me to a document whih
clarifies the matter?
-- 
Leonard Evens len math northwestern edu
Professor Emeritus, Department of Mathematics, Northwestern University