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.