grep -A 2 -B -2 "The mail system" mbox_file

-B is for privious lines  , so no need to give -negative value.

grep -A 2 -B 2 "The mail system" mbox_file <<<This will work please check

Regards Mukesh Payghan

grep -A 2 -B -2 "The mail system" mbox_file

-B is for previous lines, so no need to give -negative value.

grep -A 2 -B 2 "The mail system" mbox_file   # This will work please check