Concise example to setup an opensmtpd server with virtual domains and virtual users
Had some problems with the passwd table which where solved by following this example:
http://misc.opensmtpd.narkive.com/OpKS0DRj/table-passwd-invialid-without-the-shell

The guide is a bit too simplistic but covers some topics like spf and client configuration

Blog series to install opensmtp and dovecot with filters for spam, antivirus, dkim plus roundcube webmail

A couple of details are missing like reverse DNS (test with mail-tester):

• set the droplet name as the domain you are using for the emails (eg mail.example.com) if using digitalocean
• set the same domain in /etc/mailname

Interesting example to configure opensmtpd and dovecot, with virtual users and domains stored on a shared sqlite database