If you only want to allow certain attachments for one or two domains but still block them for all other domains, then you need to do the following.
Copy /usr/mailscanner/etc/filename.rules.conf and /usr/mailscanner/etc/filetype.rules.conf (and archives.filename.rules.conf and archives.filetype.rules.conf if appropriate) to create new conf files, for example filename.special.rules.conf and filetype.special.rules.conf. Edit the new conf files to allow the files you wish to permit through.
Then edit /usr/mailscanner/etc/rules/filename.rules.rules and filetype.rules.rules and modify the line for the domain(s) you wish to use the new ruleset, like this:
FromOrTo: domain.com /usr/mailscanner/etc/filename.special.rules.conf
and
FromOrTo: domain.com /usr/mailscanner/etc/filetype.special.rules.conf
(Each of the above lines should be all on one line in the rules files.)
Information about determining why an attachment has been blocked and modifying the filename and filetype configuration files can be found here:
http://www.configserver.com/techfaq/index.php?faqid=12
Restart MailScanner after you have made any changes to the rules files or filename/filetype configuration.
