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.
Unfortunately HackMD rejects the anonaddy.com TLD, so I've had to use my "real" address there, but so far everywhere else it works fine. A clever friend realized you can register a new github account with an anonaddy address and use that to connect to HackMD. Smart.
Great service. Free tier is great. Will probably end up paying and adding my own domain for the odd site that rejects theirs.
The only feature I'd like is greater bandwidth allowance per month on the lite plan. Current limits are 10MB per month free tier, 50MB on $1/mo lite tier, and unlimited on Pro. But fair enough.
One of the best things about AnonAddy is that it allows you to create aliases on the fly. So, I hardly even need to visit their website, browser extension or anything.
[0] https://anonaddy.com/blog/sending-email-from-an-alias-and-up...