Users are having problems with eMail. A typical problem is that a single user from a single domain is having trouble sending to another single user in another domain. Others within the domain can merrily send eMail to each other, as well as the affected users. Go figure.
You can learn a lot about how smtp mail works by using telnet to manually create the messages. A simple example is illustrated here. A more complex example, when your smtp server requires that you log in to send mail, so that it doesn’t perform an unauthorized ‘relay’ of mail, is located here. In this latter case you need to submit your login name and password encoded in Base64. So, a handy-dandy Base64 encoder decoder is located here.
Two other resources: