If you know the subject of a message, you can search the entire message store for messages with this subject.
Go to Settings → Database → Query
Alternatively, at a command prompt in the VPOP3 directory run
psql
(The password is 'vpop3pass')
Then, type
SELECT messagetime, internaltime, subject, fromaddr, fromname, tolist, attachments, spam, size, username, folder FROM allmessages INNER JOIN foldermessages fm USING (msgid) INNER JOIN folders f ON f.folderid=fm.folderid INNER JOIN users.users ON userid=usernumber WHERE subject ilike '%<subject to search for>%';
Replace <subject to search for> with the text to search for in the subject. Note that this only works if the message subject was in ASCII. If it contains non-ASCII characters or is encoded using BASE64 or Quoted-Printable encoding, then it may not find those messages.
N.B. VPOP3 does not need to be stopped while you do this