I was recently asked to complete a survey about the forthcoming Thundermail Pro service and realised that should have added an idea about integrating Firefox Relay.
Since users will be able to use custom domains with Thundermail, it makes sense to offer integration with Firefox Relay. Users could either
1. Create Relay aliases like favouritecafe@mydomain.com or abcd1234@mydomain.com
2. Create a sub-domain Relay alias like favouritecafe@mail.mydomain.com or abcd1234@mail.mydomain.com
Proton Pass let's you do option 2 and this works really well for me, but worth considering as Thundermail gets off the ground. Something I like about this (and Firefox Relay) is the ability to create aliases "in-the-moment" in a newsletter web form or similar.