ISP’s blocking Port 25 (SMTP)

Why the heck is it that ISP’s feel the need to dictate to us the ports we should and shouldn’t make use of?

I say this having just spent near-on a week of frustration wondering why the BeachyUK server hated me and refused to send my mails! After finally finding some time to sit down and diagnose the issue, I’m really irritated that it seems my wonderful new Broadband provider (O2) have taken it upon themselves to block Port 25, to encourage me to send mail via their servers.

Quite apart from the fact that the choice of server I send my mail through is my own decision, I fail to see the benefit here to O2, they’re actually encouraging me to put more load on their SMTP server rather than use my own… hmm, maybe they have an interest in the mails I’m sending 😉

So for anyone out there with the same issue with O2 Broadband (or another overly obsessive ISP), if you have control of your server, just change your mail sending port to an alternative port (and do the same on the server that’s receiving the mail, and hey presto, you’re back in business, despite the efforts of your ISP!)… otherwise, you’ll need to use your ISP’s relaying SMTP mail server to send mails, or you can use your mail servers SSL port, and send using SSL.

One finally thing actually… searching the Internet, I found a lot of sites (in fact, even O2’s help section), saying to use “” for outgoing mail if you’re an O2 Broadband customer, this however is wrong for this use. The afore-named domain is for O2 mail users. If you’re looking to relay mail on behalf of your own domain you need to use “” (no authetication required, and the mail will be seen to come from your domain name).


I'm passionate about technology, and particularly helping people make the most of it. I've spent the last 30 years helping others make the most of technology. My career started in IBM, but I choose to move into smaller business environments, to use a breadth of skills, and help businesses step change their IT services. My skills range from user based technology, through business systems (applications) to infrastructure. I also have a long background in IT security. I focus on what I consider to be "productive technology", i.e. adding genuine value to peoples lives. I'm not a big gamer, and don't hold much interest in what I consider to be disposable consumer technologies. During the day, you'll find me consulting with businesses or heading up an IT department. At the weekend, you'll find me sat at my Linux PC, writing PHP or Python code, or trying to help others on Twitter, this blog, or my YouTube channel: Artexic.