Attempting to pull in this thread a tad, there are relatively simple measures that can be taken to bring a private mail server into compliance with gmail, Amazon, Microsoft level mail server protocol and authentication. Its not just gmail. The simplest measures are done with DNS and TLS. Most of the mail that I see routinely falling into spam folder is from what appears to be spoofed domains. Many of these are legit messages that dont have a properly configured DNS record, preventing the receiving server from authenticating the FROM domain as owned by the sender. A simple fix.
Well, even with proper DKIM mail and SPF records, Google still sometimes shafts my mail. No idea why, no one to talk to on how to make it better, no options other than "Get a Google mail account".
Life like that.
