(I work on Firefox Relay.) You can set a mask to blocked, after which it will no longer forward emails to you. If you do this before deleting the mask, they wi

(I work on Firefox Relay.) You can set a mask to blocked, after which it will no longer forward emails to you. If you do this before deleting the mask, they will not re-appear. Additionally, we recommend you to use the random (i.e. @mozmail.com) rather than custom (i.e. @<yoursubdomain>.mozmail.com) addresses if you can, since those are more anonymous, and those will also never re-appear. That said, we do hear this complaint, and we do have plans to prevent this from happening in the future. It's just hard to balance with a lot of other work (such as the Firefox integration).