port question

[Bruce Marshall]

On Sunday 23 August 2009, Tony Alfrey wrote:
> Yes, thanks, it is very clear! I'm just wondering about the actual
> packet of bits that is assembled at each layer.  My understanding is
> that the port ID is part of the bit stream that makes up the packet.  If
> this packet starts out from ME where I (my mail client) put in an ID of
> port 587 into the header that precedes the data bits, does an additional
> piece of header get added (perhaps at the sending SMTP server end) with
> an ID of port 25 so that it makes it to the receiving SMTP server?

I suspect that your ISP is relaying the mail to wherever it is going....  They 
receive the mail, then send it on....  more than just changing ports on a 
packet.