How to disable Mailscanner for outgoing email only

You need to use a rules file. If you haven't already got one, modify MailScanner.conf so that

Spam Checks = %rules-dir%/spam.scanning.rules

Then create a file in the rules subdirectory called spam.scanning.rules and add the domains as follows:

To: *@mydomain.com yes
To: *@myotherdomain.com yes
FromOrTo: default no

The last one is a catchall to not scan domains that are not listed.

The key here is using To: instead of FromOrTo: to prevent outgoing email from being scanned for spam.

Stop and restart MailScanner after making any changes.