I realize that. What I'm trying to say is that I like to create 'john.doe+projectA@example.com' and 'john.doe+projectB@example.com' accounts with some services.

I realize that. What I'm trying to say is that I like to create 'john.doe+projectA@example.com' and 'john.doe+projectB@example.com' accounts with some services. Checking for the existence of any 'john.doe@example.com'-like accounts would mean I have to register an entirely separate email account or set up (another) email forwarder/alias.