[Dovecot] qpopper -> dovecot migration : which pop3_uidl_format?

[bob passendon]

Hello,
I have read in lots of places about how I should use
pop3_reuse_xuidl = yes
to prevent pop3 users having to re-download mail (and duplicate their
e-mail) after a migration.

This fails with the message:
 pop3_uidl_format setting is missing from config file

Which setting for pop3_uidl_format should I set to prevent the e-mail being
downloaded again?