Are you getting this error when trying to email in Wintix?
SMTP Error: 553 Sorry, that domain isn’t in my list of allowed rcpthosts (#5.7.1)
One of our clients reported that he suddenly started getting this error message when sending email tickets. Regular emails go through okay.
You may want to read more here: http://knowledge.3essentials.com/web-hosting/article/84/ERROR-553-sorry-that-domain-isnt-in-my-list-of-allowed-rcpthosts-
Cause: This error message is reported by your email client such as Outlook or Outlook Express because the check box labeled My Server Requires Authentication is not checked. Your email client attempts to send the email to the server without providing a password, hence the server replies that you are not allowed to relay without proper authentication.
Resolution: To resolve this issue check the checkbox next to My Server Requires Authentication.
The instructions for the checkbox differ slightly depending on whether you use Outlook or Outlook Express. (See link above.)