The conventional way for a mail client program to send e-mail is using TCP port 25, which is also the port used by mail servers to talk to each other. But port 25 is widely abused by malware to spread worms and spam. As a result, many ISPs are restricting its use.

Most users shouldn't have a problem anyway. If you only send mail using their ISP account, you can probably still send it using the ISP mail server. But if you want to send mail using the mail server for your office, or perhaps a hosted mail server for a domain you own, you could have problems.

There are a few potential solutions. If you want to use your company's mail server, perhaps you should be using a virtual private network to connect to the company network and mail through that. The other mail server may also support web mail, and since all that goes through port 80 (the http port) the ISP won't block it.

But there is another SMTP submission port, port 587, which almost all mail server supports. In fact, according to the relevant standard, port 587 is the preferred port for mail submission from clients. But even if the mail server software supports it, it may or may not be turned on. You need to check with the administrator or hosting service. Not all hosting providers support port 587, but we do!

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