Fix message id for Postfix in Message Operations

Mailscanner adds a dot and a 5 digit random number to Postfix message id; this message id is used in Message Operations as variable names in radio buttons, but PHP doesn't allow dots (.) in variable names and changes them with underscore (_) [http://php.net/manual/en/language.variables.external.php].
This fix checks if $mta is Postfix and changes back underscores to dots making Message Operations works.