SMTP Error (451): Failed to add recipient “firstname.lastname@example.org” (Temporary local problem – please try later).
My server Operating System: Centos 7.8
Problem with Exim and this Problem come after update exim on CentOS 7
Solution: simply Downgrade Exim Updated version to compatible version
$ wget https://github.com/KashifHK123/server-fixes/raw/master/vestacp/exim-4.93-3.el7.x86_64.rpm $ rpm -Uvh --oldpackage exim-4.93-3.el7.x86_64.rpm