If by Microsoft you mean Exchange server, not being compatible will be a literal deal breaker in doing business.
An awful lot of big players won’t host their email on someone else’s service, so gmail is just not an option. If you deal with any business using primarily windows, there will also be a huge probability it’s running a Microsoft stack in full, from account management, email to file hosting.
"Your private server can afford to not be inter operable with Microsoft, Yahoo, and AOL, but not Google"
Seems we are not living on the same planet.
What do they do from an SMTP point of view that's different to the usual/RFCs?
> Your private server can afford to not be inter operable with Microsoft, Yahoo, and AOL, but not Google.
Citation needed. This is the opposite of much of the world of work, where emails often both originate and terminate in Exchange Servers, not touching gmail servers.
Simply because if they start pushing some format, everyone must go along with them. Your private server can afford to not be inter operable with Microsoft, Yahoo, and AOL, but not Google.
And you can rest assured that email you send, anywhere, will eventually be forwarded to someone on a Google server. If not directly, than via Google Apps.
But in literal terms as well: It happens all the time that I ask for someone's email, and they respond with the username and assume the "@gmail.com".