You can search /var/spool/mqueue, find the bouncing email IDs and delete them.
To do so, run:
find the ID at the LHS of the output:
Code:
NAA21610 16176 Tue Aug 3 13:26 MAILER-DAEMON
(Deferred: Connection refused by x.x.x.x )
and delete the corresponding 2 files:
Code:
rm /var/spool/mqueue/?fNAA21610