On 23.7.2007, at 16.47, bob passendon wrote:

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?

If all your existing mails have X-UIDL: header then those are used
for existing mails, and pop3_uidl_format is used for new mails. So it
doesn't matter what you use there.