I'm only just starting to look into email on Tor - and so far I'm only finding email hosts whose sites (including webmail login) are hosted on .onion addresses, and whose POP, IMAP and SMTP servers can clearly be connected to in the .onion namespace (even within your thunderbird mail client) - but how about having a .onion address as the email address itself?

Is it possible to have an email address like [email protected], and have full .onion <--> .onion email address correspondence?

I can think of obvious advantages, such as avoiding privacy and even security issues associated with referencing to the standard DNS infrastructure (with sniffable DNS lookups and hijacking), as well as potentially relying less on the standard (and abusable) SSL/TLS certificate validation system, and just further obfuscation by being insulated entirely within the Tor network as a whole - especially if the .onion email services were configured solely to be a hidden service and not connect to standard DNS at all.

Has it been done before - are their services that do indeed offer it now - or has the idea not actually been explored before?

There still is a use and need for non-real-time, long-storage (until retrieval), text correspondence, so OTR instant message (or say IRC using persistent BNCs) surely cannot satisfy the need for all 100% Tor, DNS-less, reliable text communication of this type.