Hello,

please have a look at this good explanation how aliases work (the first answer) : https://www.odoo.com/forum/help-1/question/how-to-setup-email-alias-111570. 

Basically, if you have a properly configured catch-all, the emails would be there. However, if no alias is defined (or no properly configured meta data), Odoo would ignore such emails. It just can't know where to put those messages. And you can't define an alias for each possible combination.

The only solution (I guess the only) is to re-define the method message_route of mail.thread. Currently, undefined messages are ignored, but you may apply any sort of action on a code level. For example, we faced a similar issue: not all users reply to messages, but create a new email. Such emails are lost in Odoo. That's why we have re-defined the method to put such emails in a special menu for further re-direction. Regretfully, the app is paid - https://apps.odoo.com/apps/modules/10.0/mail_manual_routing/, but you may rely upon the same logic while developing your tool.