I’ve been using my own mail servers for decades. After all, in the 80s I was helping to run the messaging systems at NASA’s Goddard Space Flight Center and let me tell you, back then it wasn’t easy! Today, thanks to simple email servers such as CapeSoft Email Server, hMailServer, and Zimbra, just about any knowledgeable user can run an email server. Heck, if you’re a cut above a power user, you can even run OpenExchange and fully support Outlook users without breaking a sweat. If, that is, your Internet Service Provider (ISP) allows it.
As a Slashdot reader recently discovered, many ISPs won’t let you manage your own mail server. Specifically, they block port 25, the Simple Mail Transfer Protocol (SMTP) port, which is used to send mail. If you can’t send mail, there’s no point in having a mail server.
While some Slashdot readers were outraged by this, there’s nothing new here. Comcast, AT&T, and Cox, to name a few ISPs, block port 25 as a matter of course and have done so for years. Why? As one person put it, “Most ISPs block outgoing port 25 because 99.99% of that traffic is viruses or malware trying to send spam. Even more email services block all dynamic pools used by major ISPs for the same reason.”
He is of course right. Many Windows malware and botnets attempt to send spam through port 25. Indeed, most botnets are designed expressly to send spam. Indeed, last year, Daren Lewis, a security analyst at Symantec, discovered that 80% of all spam is sent by these 10 spam botnets that use approximately five million Windows PCs to send 135 billion spam messages. per day. So who can blame most ISPs for simply blocking port 25?
Well, the few users who know enough to run their own mail servers from their SOHO (small office/home office) and small businesses can and should blame them. If you’re like me, Gmail might be perfect, but you like having real control over your mail, mailing lists, etc. So what can you do?
Well, for starters, you can avoid using port 25 and use port 465 instead for secure SMTP. It is much rarer, but not unheard of, for ISPs to block this port. It also makes your outgoing email much harder for any potential spy to read.
My own answer for many years has been to run my own SMTP server from a hosted server. If, as has happened, my ISP tries to block my email clients from using port 25 or 465 to access them, I call them, work my way through tech support two levels above usual technology. support suspects and request the opening of ports. So far I beat 1,000 with this approach.
If for some reason they didn’t, while I was looking for a new ISP, I would change my mail server and client ports to another port, say 2525, and use that instead .
If you’re not sure if it’s your ISP, or maybe it’s due to a firewall or mail server error, I highly recommend using MXToolbox, an online set of Email problem analysis tools to find out where the problem is occurring. . If it’s your ISP, call. If not, there are far too many possible issues for me to try to give you even an overview of what might go wrong. Chances are if you’ve sent mail without any problems and your mail server and/or your clients can’t connect, it’s your ISP and they just blocked one or more of the SMTP ports. Good luck!