This will list failed outgoing messages from the last 3 days (with the default VPOP3 settings)

At a command prompt in the VPOP3 directory run

psql

(The password is 'vpop3pass')

Then, type

select created,returnpath,subject,username || '@' || domain as recipient, latesterror 
from outqueue.outrecipients inner join outqueue.outmessages using(msgid) where
failedtime is not null order by created desc;

N.B. VPOP3 does not need to be stopped while you do this