Create one Amazon ec2 free-tier or nano VPS instance running with Iredmail. http://jeffreifman.com/how-to-install-y … cloud-aws/
(I skipped the part about mailgun and used Amazon's own SES for outgoing mail. Also FYI mailgun does not forward incoming email to your server, only email addresses, see here: http://serverfault.com/questions/839437 … 869#839869 And that is why I opted to use mxguarddog to filter and route incoming mail to the mail server.)

Disable all antivirus and spam scanning
http://www.iredmail.org/docs/completely … assin.html
http://www.iredmail.org/forum/topic1249 … -spam.html
Disable greylisting
http://www.iredmail.org/docs/manage.ire … completely

Use mxguarddog to route all incoming mail to your mail server. (Make sure to make firewall rules only allowing smtpd mxguarddog IP addresses over port 25) https://mxguarddog.com/faq.ip_list/ (use the built in "security groups" of amazon, don't modify the built-in iredmail rules. ec2 security groups will be the first line of defense.)

Use Amazon SES as a relay to send all your email with postfix. http://docs.aws.amazon.com/ses/latest/D … stfix.html

In effect, I have a scaleable mail server where I can provide my clients email hosting with unlimited space (costing me .10 / GB) for storage space using an amazon EC2 VPS instance (couple bucks a month).

The mail server is efficient and operates on about 200MB/RAM by outsourcing virus and spam scanning to Mxguarddog (receiving) and Amazon SES (outgoing), (as well as receiving "reputation" for going through an established mail send point), and finally all is passing DKIM and SPF verification. And don't forget to use free letsencrypt for SSL http://www.iredmail.org/forum/topic1250 … nginx.html

I consider the project a success, but I do wish I could find a more "enterprise" grade mail routing/filtering solution for incoming mail than mxguarddog.