port question

[Bruce Marshall]

On Sunday 23 August 2009, Tony Alfrey wrote:
> I must select Port 587 for the SMTP server settings when I use my e-mail
> client (SMTP client?), otherwise I cannot get my packets to be accepted
> by my SMTP server (earthlink) or allowed for transmission to my SMTP
> server by my isp (comcast).   But you (and other) sources say that Port
> 25 is always used for internet e-mail.  Does this mean that the port ID
> in the packets is changed from port 587 to port 25 (or additional port
> 25 headers added) by the transmitting SMTP server?

Let's talk in big generalties here....    Port 25 is the common SMTP port and 
is almost always used. to RECV mail at an ISP or in some cases and end user 
running an SMPT server.

Yes, a lot of ISP's will use port 587 (dunno why other than to cut down on 
people trying to break in on 25) for their SMTP servers but they will usually 
also require some means of authentication...   to avoid spammers from using 
their server.

End users (you and the two others) would NOT be receiving mail on port 25 but 
you *do* use port 25 (or 587) to send mail to your ISP for forwarding.  So 
it's  port 25 OUTGOING from you, but never port 25 INCOMING for you.   And...   
if you tried to put up an SMTP server on your machine, RECVing on port 25, you 
couldn't do it because they would block any packets coming to you on 25.

So the big generality is that  ALL ISP's use port 25 to receive mail from you 
and to forward that mail to other SMTP servers.  Many ISP's (but not all) 
would block port 25 calls to their end users to prevent them from running 
their own SMTP servers.

As always, there are some exceptions to the above, but the above is the 
general rule.