A string defining the mail user, and optionally group, to execute external commands as. This maps to the user parameter (pipe(8)).
Default: 'vmail'.
Example: 'vmail:vmail'
##### `mailman`
A boolean defining whether to configure a basic smtp server that is able to work for the mailman mailing list manager.
Default: false.
##### `maincf_source`
A string defining the location of a skeleton main.cf file to be used. The default file supplied is blank. However, if the main.cf file already exists on the system the contents will NOT be replaced by the contents from maincf_source.