sacred-rat

Looks like it came out Feb last year[0], so it has been out for awhile. I just learned about it recently.

[0] https://anonaddy.com/blog/sending-email-from-an-alias-and-up...

I have been using AnonAddy[0] for this, with great results. I initially used Firefox Relay, but switched to get more than 5 aliases. AnonAddy also recently added support for replies.

[0] https://anonaddy.com/

It's back up now.