In my case I was trying to create an account on the Linux Mint Forums [1]. The confirmation email never arrived, which was very confusing to me.
[1]: https://forums.linuxmint.com/
After a couple emails with the admin, they told me this:
> The forum tried sending you the activation email but it'd rejected by the Firefox relay with this message:
<...@relay.firefox.com>: host
inbound-smtp.us-west-2.amazonaws.com[54.240.252.212] said: 550 5.7.1 TLS
required by recipient (in reply to RCPT TO command)
> This is a known issue of the Firefox relay: https://github.com/mozilla/fx-private-relay/issues/757. I'll check but I think TLS is not under our control, same as in the linked issue.> For now I think you'll have to use a different email address.
So while it looked promising, sadly the next day I was already back to using gmail addresses...