Install gross by entering the following commands in the terminal:
sudo apt update sudo apt install gross
Description:
fast and efficient greylist server with DNSBL support
Gross is a resource efficient greylist server written in C that supports greylisting and/or blocking based on DNSRBL so it will not impact legitimate mails . It also contains a milter implementation and natively supports Postfix, Exim and Sendmail.
Homepage: http://code.google.com/p/gross/
Version: 1.0.2-4build1
Section: universe/mail