If when sending an e-mail using your e-mail client (such as Mozilla Thunderbird or Microsoft Outlook), it fails with an error message containing '553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)' then you haven't enabled authentication for outgoing e-mails. You will need to go back into your account setrings and tell the program that you need to provide a username and password for outgoing (SMTP) e-mail (which should be the same as the username and password for your incoming mail account).
The message is rather cryptic as the way the mail is handled is different depending on whether or not is was submitted with authentication. The rcpthosts file is the file used by qmail, our e-mail server software, to tell it which domains it is responsible for and therefore which it should receive mail for from other servers. Without authentication, the server can only accept mail for these domains.
As we allow clients to use the server to send out e-mails, we need to make sure that only our clients can do so - to allow anyone to send mail though it would put the server at risk of becoming a SPAM relay. Therefore, to send outgoing e-mails through the server you must set the account properties to also provide your username (i.e. the full e-mail address) and the password with it sends e-mails.
