Using the access rules specified in <listdir>/control/access to perform access control to the list. This is done before headers are stripped, so one can create allow rules based on headers that are later stripped.
Headers specified in <listdir>/control/delheaders are deleted from the mail.
Headers specified in <listdir>/control/customheaders are added to the mail. This could be headers like List-ID: or Reply-To:
In case there's a mail with a recipient delimiter it's not a regular list mail. Processing of these happens in mlmmj-recieve as well. Examples of such are subscription requests, mails to owner etc. It will base it's recipient delimiter detection on the Delivered-To: header if present. If not, the To: header is used.
If the list is moderated, it will happen in mlmmj-process.
When processing is done, it will invoke the needed binary according to whatever mail it is. If it's a subscription request it will invoke mlmmj-sub, if it's a regular list mail it will invoke mlmmj-send.