Vista sending mail on Port 587 only

A Vista PC which was sending SMTP mail fine on Port 25, suddenly stopped.  I have fixed the problem by sending on Port 587 - but what I want to know is why?  I know this much:
1.  It's not the AV/Firewall (PCCillin2007) as turning the software off makes no difference.
2.  It's not the WIndows Firewall  -it is not turned on.
3.  It's not the firewall on the router  -XP PCs on the same network can send on Port 25 with no problem.  This also shows that it is not the ISP's mail server.
4.  I have tried Telneting into the SMTP server.  It hangs until you hit Control C - then you get in to the server and communicate with it.
Can anyone tell me what is going on?
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KenneniahConnect With a Mentor Commented:
Bottom line though, it that more ISP's are starting to follow that RFC and splitting smtp into the 2 seperate functions listed in RFC 4409. Vista is more compliant with this newer standard and is restricting secure (to use the term from the rfc 'submissions') to port 587, and only allowing 'relay' to port 25. My guess is that your ISP's smtp server wasn't using this standard until recently, hence the change.
chris_shawAuthor Commented:
Thanks for the reply Kenneniah,

I had read the post you refer to prior to posting my question, and was why I tried Port 587 - thank you.

However, I'm not convinced that this is the cause for the following reasons:
1. The RFC seems to suggest that this processing is carried out by the SMTP MSA - but if this were the case, then I would have thought that XP PCs sending mail from the same LAN to the same SMTP server on the same ports would have experienced the same problem - but they don't.
2. Isn't Outlook just using the SMTP server as a 'relay' ?  In which case there shouldn't be a problem should there?
3. The message is not being rejected by the server once connected.  The RFC seems to suggest that it would refuse to relay and would return an error in certain situations.  But there is no error being returned.  What is happening is that the Vista PC cannot successfully establish a connection to the SMTP server (see my note above about using TELNET).
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