553 sorry, that domain isn't in my list of allowed rcpthosts

This error signifies that you aren't authenticating when sending emails.

Cause:
Your email client attempts to send the email to our 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”

Instructions:

Outlook Express: 
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 and up:
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

Tags: #5.5.3 - chkuser, rcpthosts, sending emails