24th Jun 2007
Does your ISP block port 25?
We read recently about a business traveler who could not send emails at all whilst traveling abroad with his laptop. The problem was deceptively simple. The traveler used Outlook, and the ISP he was using blocked port 25.
As he could receive emails fine, it was clearly not a problem with his Outlook spam filter. Rather, it was a server-side issue: some ISPs restrict port 25 to some extent, or even entirely. AOL users, for example, cannot connect to port 25 at all, the block acting as a de facto spam black list. Simply put, port 25 is the default connection port for sending emails, and thus one used by a lot of spam tools and spam solutions. By blocking it, ISPs hope to stop these robots from taking advantage of their network, whilst letting regular users fix the problem manually. For someone like our business traveler, it’s easier said than done when they have no idea what the problem is.
Of course, if you use web mail, you avoid the problem entirely! If you don’t, here is a simple solution if you find yourself unable to send mail and suspect it may be a port 25 issue.
Using Outlook or Outlook Express? Spam may not be a problem, but your connection might be. Access your Outgoing SMTP server and add :587 to the end, e.g. sendmail.yourserver.com:587. 587 is left open by most providers when 25 is blocked.
Still no progress? Your ISP may have blocked port 25 entirely, even from outgoing connections, or your mail server may not exist. Contact your ISP and ask them for further details on a valid port to send legitimate email.
Port blocking may be a noble attempt to get rid of spam, but it can also be a major inconvenience. Make sure you don’t suffer from this problem.
Posted by Tom under 
Tips & Tricks 
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