VERP info not added to mail headers?
In the Mail Accounts entry you set up, you need to set bounces+ as the localpart.  Everything else looks good.

In your examples, you have b.94.15.1ff1d88a496eafde@mydomain.com as the return-path in the the first one and b.100.16.af889b4c23dd0331@mydomain.com in the second.  CiviMail just needs to know to tack "bounces+" on the front of those addresses, and that's where the localpart comes in.