Email is one of the bastions of the decentralised Internet and we should hang on to it.
Every day more and more people are moving to Gmail/Hotmail/Outlook and while I do understand the reasons, it also puts more and more power into the hands of these providers and the little guy (us) gets more screwed (like marked as junk by default by them :< )
Having said that, here's my check list for successfully delivering email:
- make sure your IP (IPv6) is clean and not listed in any RBL, use e.g. http://multirbl.valli.org/ to check
- make sure you have a correct reverse dns (ptr) entry for said IP and that ptr/hostname's A record is also valid
- make sure your MTA does not append to the message headers your client's IP (ie x-originating-ip), messages can be blocked based only on "dodgy" x-originating-ip (see eg https://major.io/2013/04/14/remove-sensitive-information-fro... )
- set up SSL properly in your MTA, there are so many providers giving away free certs nowadays
- SPF, DKIM, DMARC - set them up, properly, this site can come in handy for checking yourself https://www.mail-tester.com/
- do not share the IP of your email server with a web server running any sort of scripting engine - if it gets exploited in any way usually sending spam is what the abusers will do
- last but not least - and while I loved qmail and vpopmail - use Postfix or Exim, they are both more fit for 2016, more configurable and with much, much larger user bases and as such bigger community and documentation.
HTH