Not true. Google's G Apps, Amazon Work Mail, and even Zimbra support the Exchange Web Services/Exchange ActiveSync client protocol.
Exchange is the de-facto standard business PIM client protocol. This shift happened maybe 10 years ago when Microsoft was forced to license these protocols and people on mobile demanded an all-in-one push sync protocol.
If you were to implement this separately, you'd have a huge mess of protocols: IMAP, SMTP, iCal/CalDAV, vCard/SyncML, LDAP. I don't think there's even an open standard to handle tasks.
Does Mail on the Mac still muck up CSV files? I did some tests a couple years ago with it and every time I sent a CSV file it would add a space between every line - only with Exchange accounts. I reported it to Apple for about 6 months and then gave up on Mail for good. An app should've be mucking up my attachments. Unacceptable. Straight up.
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