I migrated a CPanel backup file from an old server. Its approx. 1Gb of email in about 20 accounts.
All the users accounts were created, but there was no email in Usermin.
A little digging showed that all the imported emails were in the folder /home/account/Maildir/ (under each users name), and the /cur/ and /new/ folders in /home/account/Maildir/thedomain.com/ in each users files were empty.
I had to run command lines to copy all mail from each users folder from /home/account/Maildir/ into /home/account/Maildir/thedomain.com/, then change permissions, so I have recovered it but was obviously a pain to do! Only spent about an hour, but still…
The issue was with a Cpanel Addon domain (so Virtualmin sub-server)… the main Cpanel account domain also has email (approx. 260Mb) and this was migrated over just fine!
P.S. I have set the users username to user@domain because its a Cpanel import and I dont want users affected by having to change settings.