Their own Gmail client. I have yet to see one that works with normal email.
There’s barely any point in setting up your own provider now because you have to compete both with free. All the action has been around email clients which have typically been acquired and then crushed.
Google could have stuck with the protocol or presented a spec for it, rather than leaving the competition with IMAP and SMTP.
What makes sense is they dominated the market and helped put up roadblocks afterwards. EEE.