I've been running my own e-mail servers for decades. After all, back in the 80s I was helping run NASA Goddard Space Flight Center's e-mail systems and let me tell you in those days it wasn't easy! Today, thanks to easy e-mail servers such as CapeSoft Email Server, hMailServer, and Zimbra pretty much any tech savvy user can run an e-mail server. Heck, if you're a step 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) will let you do it.
While some Slashdot readers were outraged by this, there's nothing new here. Comcast, AT&T, and Cox to name but a few ISPs, block port 25 as a matter of course and they've done it for years. Why? As one person put it, "Most ISPs block outgoing port 25 because 99.99% of that traffic is viruses or otherwise malicious computers trying to send spam. Even more mail services block all dynamic pools used by major ISPs because of the same reason."
He's right of course. Many Windows malware programs and botnets attempt to send spam via port 25. Indeed, most botnets are designed expressly to send spam. Indeed last year, Daren Lewis, a Symantec security analyst found that 80 percent of all spam is sent by these the 10 spam botnets use about five million Windows PCs to send out 135 billion spam messages a day. So, who can blame most ISPs for just blocking port 25?
Well, those few users who do know enough to run their own mail servers from their SOHO (small office/home offices) and small businesses can and do blame them. If you're like me, Gmail may all fine and dandy, but you like having real control over your mail, mailing lists, and the like. So what can you do?
Well, for starters, you can avoid using port 25, and use port 465 instead for secure SMTP. It's far rarer, but not unknown, for ISPs to block this port. It's also makes your outgoing e-mail much harder for any would-be spies 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 mail clients from using ports 25 or 465 to get to it, I call them up, fight my way through technical support to two levels above the usual tech. support suspects and ask for the ports to be opened. So far, I'm batting 1.000 with this approach.
If for some reason they wouldn't do it, I'd--while looking for a new ISP--I'd switch my e-mail server and client ports to another port, say 2525, and use it instead.
If you're not sure if it's your ISP, or maybe you thanks to a firewall or mail server setting blunder, I highly recommend using MXToolbox an online set of e-mail trouble-analysis tools to get a handle on where the problem is happening. If it is your ISP, get on the phone. If it's not, there are far too many possible problems for me to try to give you even a sketch of what might be wrong. Odds are though if you've been mailing along without any trouble and then your mail server and/or clients can't connect, that it's your ISP and they've just blocked one or more of the SMTP ports. Good luck!