Symptoms And Errors:When attempting to send a email using your 3Essentials hosting account, your email client may report the following error message:
"553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)"

The full error message may appear as:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'emailaddress@domain.com'. Subject 'Email Subject', Account: 'Your Account Name', Server: 'mail.yourdomain.com', Protocol: SMTP, Server Response: '553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)', Port: 25, Secure(SSL): No, Server Error: 553, Error Number: 0x800CCC79

However, receiving email works correctly.

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"

Outlook Express: (reference image below)
Open Outlook Express > Click Tools > Accounts > Click the 'Mail' Tab > Click Properties > Click The 'Server' Tab
check the checkbox next to "My Server Requires Authentication"
Click Ok > Click Close

Outlook 2003 (reference image below)
Open Outlook 2003 > Click Tools > E-Mail Accounts > View or change existing e-mail accounts
Select the email account > Click Change > Click 'More Settings' > Click the 'Outgoing Server' Tab
Check the checkbox "My outgoing server (SMTP) requires authentication"
Click OK > Next > Finish