TL;DR you can send emails from .onion addresses if you want, but no clearnet server is going to accept them.
So when you send an email, you can actually put whatever you want in the from
header. I could send an email that says from "[email protected]". The protocol doesn't care.
Do you know who does care? The email server you're sending messages to, because spammers and scammers love to try and send email with fake from
addresses.
So, there's an entire verification system in place that involves looking up public keys from the website that the email claims to be from. (this is a gross over simplification. Look up SPF, DKIM, and DMARC for more info). The problem is you can't even reach .onion
sites from the clearnet to do the lookups. So no email servers would be able to validate your address is legitimate and so would drop it as spam.