It's  nearly  impossible to  run  a  private email  server  anymore,
without running  afoul of  the big providers'  proprietary filtering
schemes that will reject mail from  your server at random.

Once upon  a time, it  was enough  to check the  public blacklists.
Those are worthless now save for  the obvious spammers. Twice in the
past few years I've had to  change clients' mail server IP addresses
to get mail  flowing again.  I no longer recommend  running your own
mail server. You will be assimilated.