The following method applies to SmarterMail 8.x - 12.x

Follow these steps to add a domain or an email address to the SMTP blocked senders list:

Log in to SmarterMail as the system administrator.

Click the Security icon.

Expand the Advanced Settings folder in the navigation pane.

Click SMTP Blocked Addresses. A list of blocked addresses will load in the content pane.

Click New in the content pane toolbar.

In the Blocked Address field, type the domain or email address that you wish to block.

In the Block Type field select Incoming.

Click Save.