When attempting to send mail through your account's SMTP server, the following error may occur:
This error is not isolated to one mail client and the result is always the same, the messages in the Outbox are not sent.
Our servers are designed to allow only authenticated POP account users to send mail (i.e. the "send from" address must be an existent and authenticated POP account). This prevents spammers from sending untraceable messages through our SMTP server.
By default, email clients do not submit any login information for validation while sending mail, so you are not an authenticated user unless you check for mail, first. When there are messages waiting to be sent, mail clients try to send the messages first, and then check for new messages. This sequence is what generates the error.