Almost all mail client program send e-mail using 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.
Hang on – it will make sense in a second:
Most people having a setup where they are on an Internet Service Provider (ISP) like ATT and have an email address like “MyEmail@ATT.net” You connect to the internet using ATT and send mail through the ATT server – no problem.
But if you have your own domain email address or an email address from another ISP you may hit a snag. In these cases you are connecting to the internet through your ISP (like Comcast, SpeakEasy, ATT, etc) and trying to send email through your third party host like “MyEmail@SuperCoolWebsite.com”
Your ISP says “Hey! There is someone on my network trying to send email through someone elses address! This might be spam or malware!” You can receive mail, but you can’t send mail.
Recently ATT and Verizon both made changes that affected outgoing mail. Here’s the fix:
If you cannot SEND MAIL
Other email programs will have similar settings. Look for PROPERTIES or PREFERENCES and find the outgoing mail server option.
One other option to check is that you are authenticating before sending. In other words. logging in with your username and password to both check and send email.
In Outlook the options window looks like this:
This tells Outlook to send your username and password before trying to send mail. If you have problems try enabling or disabling this setting.
The screen shots used in this how-to are my actual settings and they work just fine with SpeakEasy and ATT.