In the onion v2 days we had underwood2hj3pwd.onion. There were half a dozen other onion email providers but Underwood was the only one that did not have a clearnet email alias (IIRC). That was a useful feature because you could distribute an onion address to a MS Outlook or Gmail user and they could not use it to share their correspondence to you with Google or MS in the loop. They had just two options: step off the ad surveillance platform or not contact you at all. That option died with Underwood.
The other onion email services all have a clearnet translation. So if (for example) I give a gmail user this address:
foo@yllvy3mhtamstbqzm4wucfwab57ap6zraxqvkjn2iobmrtxdsnb37dqd.onion
and they are motivated to reach me, they can figure out that the corresponding clearnet alias is foo(/at/)onionmail.info and then they can use that address to send me a msg that is then shared with their surveillance advertiser. And worse, that’s less effort for them than obtaining an onion email account.
So what I do now is give an XMPP account. Since Google has abandoned jabber and MS never partook, XMPP avoids Google and MS. But XMPP is not a drop-in replacement for email. OMEMO is glitchy/buggy with pitfalls.
I would like to offer an email option. Ideally, an onion email service would offer a clearnet alias that cannot be determined from the onion address, which implies a different userid string.