Exchange
ExchangeFinder
Gmail
Abusing /gxlu for user enumeration
https://blog.0day.rocks/abusing-gmail-to-get-previously-unlisted-e-mail-addresses-41544b62b2
Script it.
tl;dr - https://mail.google.com/mail/gxlu?email=<valid account> 204 response has a Set-Cookie header that an invalid account doesn’t.
This should still work as it has been wontfix for years, but I haven’t tried it in a while.