ISP’s blocking Port 25 (SMTP)

Last updated on March 13th, 2021

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).

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