This is a very common error message and is seen in most e-mail programs. There are variations of this message, such as Relaying prohibited or Unable to relay, but they all mean the same thing: the server you are trying to connect to will not allow your mail to be delivered.
There are two reasons why this would occur:
1. Incorrect login details:
- If you are sending through mail.yourdomain (i.e. the same server from which you download your mail), then the server will expect a user name and password.
- You will need to enable authentication for outgoing mail in your e-mail software before any mail can be sent.
- A number of guides on checking your outgoing mail settings are available here.
2. Using the incorrect SMTP server:
- If you have an Internet connection with Company A, then you will generally be allow to send mail through their SMTP server.
- However, if you change to a different Internet connection with Company B, then the SMTP belonging to Company A will not allow you to send mail. This is done purely to avoid abuse by spammers.
- You will either need to use the SMTP server belonging to Company B or switch to mail.yourdomain.
Note: It is recommended that you send through mail.yourdomain, as this server will allow you to send mail no matter which Internet connection you use.