The 5 most common reasons for this are:
1. You don't have an account or a POP3/IMAP account hasn't been set up
You must create a POP account before sending emails and use the username and password details to send emails. If you do not have the option to create a POP3 account then please contact your reseller directly if you think you should have email resources available on your account.
2. Wrong Password.
You are using the wrong password for your email.
You must authenticate before you can relay (send) mail through our mailservers. This means that you must forward a valid username and password for your email account before our mailserver will accept your connection - this is called SMTP AUTH.
We use this system so that only authorized users can relay mail through our servers. If we didn't do this it would leave our server open to abuse by Spammers.
4. SMTP Port Blocked by ISP
Some ISP's are starting to block the SMTP port (port 25 blocking) so that you must use their email servers for sending emails. This usually isn't a problem and your email address will appear exactly the same as it already does.
Using you ISP can be quicker since you are already authenticated by being online through their system.
5. Your account is disabled or you have been automatically stopped from sending emails
It is possible that your ability to send emails has been disabled for some reason. This scenario is much less likely than the 4 reasons above, and you should check them first.
The most common reason is excessive SMTP connections within a very short period of time - for instance constantly hitting the send/receive button on your email client may trigger this protection (SMTP abuse detection) on the email server. In this case the IP you are using will be banned from connecting to the email server for several hours. Every domain will be given a "grace" period the first time this happens, but future events will result in having to wait until the ban is automatically lifted by the email server. All people connecting using the IP that triggered the protection will be unable to send emails during that period.
It could also be that your account has been compromised and has been used to send spam. If this is detected your account will have been suspended immediately, the password changed, and the ability to send/receive email is stopped. If this happens we will be in touch to discuss the matter.