I was helping a friend who has recently installed O2 (UK) broadband at his premises.
He was using Microsoft Outlook and the first thing he noticed was that – he was unable to send/relay emails via his own SMTP server setting, although his incoming emails were unaffected.
My initial diagnostic on his PC, was to check his outgoing port 25 access to various external smtp servers, as expected, I was unable to connect to any.
I went through his PC’s firewall settings, antivirus and spam filter settings, and even I investigated the router’s firewall configuration.
After almost an hour, I could not find the reason why the outgoing smtp connections were denied – until….. thanks to David Nicoll’s blog, I’ve found the answer.
[Link is now dead but read on
In essence: O2 broadband blocks all outgoing port 25 SMTP connection UNLESS you are using O2’s own mailserver, and relaying mail through their server.
Luckily for my friend, he is using Spam Arrest’s SMTP server – which allows connection from an alternative port and thus by-passed O2’s restriction
(I am not trying to advertise for Spam Arrest – just please note that you do need to be a paid user to use spam arrest’s smtp server for sending outgoing mail)
I hope this will help some new O2 broadband users looking for a solution.