In recent cPanel update to v94.0.7 you might be facing a Greylisting error which seems like:
invalid “condition” value “no
“OR”
invalid “condition” value “yes
temporarily rejected RCPT invalid “condition” value “no
That is a bug came in while recent changes to Exim to adjust the logic checks for conditions but its resulted in the error mentioned above. Now this is already highlighted with the cPanel team and they are investigating it further
Temporary Solution:
Currently you can disable the Greylisting option in WHM to fix it for the time being. In next cPanel update that issue will be fixed permanently.