port question

[Tony Alfrey]

Bruce Marshall wrote:
> 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, I see that by reading various sources.

> 
> 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)

so they say

> for their SMTP servers but they will usually 
> also require some means of authentication...   to avoid spammers from using 
> their server.

Yes, I use the earthlink smtpauth server to send mail from Port 587. 
When I am at a remote wireless hot spot, I believe that I can get to the 
earthlink smtpauth server with Port 25 but I will have to check.  Also, 
when I talk to the comcast tech support people, they tell me that they 
block port 25 from ME if I try to use port 25 to send mail elsewhere.  I 
cannot use port 25 any time I try to get anywhere with comcast.  I can 
use port 25 if I use a dial-up connection to the earthlink dial-up 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.

So since the port ID is part of the header attached to the packet by 
each layer (SMTP, TCP, IP, and so on) since it starts out from ME with 
an ID of port 587, does the port 25 ID get added so the packet can 
transfer between ISP servers?


-- 
Tony Alfrey
tonyalfrey at earthlink.net
"I'd Rather Be Sailing"