• Create a Droplet at DigitalOcean (use my link and get a month free or more!)
• apt-get update && apt-get install python-minimal git
• add droplet ip to mail.yourdomain.com and mail.yourdomain.com to MX record
• git clone https://github.com/modoboa/modoboa-installer
• cd modoboa-installer && ./run.py mail.yourdomain.com
• goto http://mail.yourdomain.com
• Edit admin password under admin > Settings
• add user@domain.tld as simpleusers in identities
• now you can login as user@domain.tld in a private browser session
• and try send an email to check-auth@verifier.port25.com to check if its working

not receiving mail?

amavis can hang for some reason if you have little memory etc

• crontab -e
• insert @hourly service amavis restart
• now amavis will restart every hour, you can change it to @daily etc
• try creating some swap space if you have issues