First Mailcow is awesome, but I am facing issue with my split domain configuration.
We have a few gmail account (exposed mx) and it forward unknown recipient to mailcow, So I have account on both Mailcow and Gmail.
My main issue is to handle the catch all in Mailcow, actually everything work if I set my domain to check all 3 
Relay this domain
Relay all recipients
Relay non-existing mailboxes only. Existing mailboxes will be delivered locally.
But that way neither Gmail or Mailcow can Handle the catch-all
I want to uncheck the last 2 but I need to add a (“blind”) mailbox for every single recipient that should be relayed but this is not documented any where.
How Can I create those blind or can I do it with transport instead
Thank you for the help and for what my opinions worth, what I am trying today can help many people to safely expand their email user without some of them staying on gmail.