iRedMail is:
1. A ZERO COST, fully fledged, full-featured mail server solution. All components are free and open source software
2. It support unlimited domains and email accounts.
2. SpamAssassin, ClamAV, SPF, DKIM, greylisting, whitelisting, blacklisting.
3. Stores mail accounts in your favourte backend: OpenLDAP, MySQL, MariaDB, PostgreSQL.
4. Supports major Linux distros

You know how difficult is to configure a fully fledged email service you have tried. iRedmail do this heavy part for you. I would recommend this piece of software to everyone. I used to configure email server manually using Postfix, Postfixadmin, MySQL, SpamAssassin, etc. Even though I have done this several times, it needs 2-3 hours for setup. But iRedmail can do all of this and MORE in few minutes, thus saves lot of time.

How to disable greylisting completely

Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will “temporarily reject” any email from a sender it does not recognize

To disable greylisting completely, please remove plugin name greylisting in iRedAPD config file
/opt/iredapd/, parameter plugins =:
plugins = […, ‘greylisting’, …]

Restarting iRedAPD service is required.