Code:
find /home/*/imap/*/*/Maildir/{cur,new} -mtime +30 -type f -exec ls -la {} +
for printing a list of files of emails older than 30 days in a console. If you see non-empty results then you should really check command you use.
For deleting files server-wide:
Code:
find /home/*/imap/*/*/Maildir/{cur,new} -mtime +30 -type f -exec rm -f {} +