I just tried to migrate one of my users from maildir to mdbox using dsync. My conversion script is checking the dsync exit code to know if the conversion goes fine or not, and surprisingly dsync returned "0" at the same time as it gave the error:

Error: Failed to sync mailbox .ta\ vare\ på ... 
(sorry, lost the rest of the error message)

Changing the folder name to mUTF7 manually made it work, but I didn't like that dsync returned success when it got this error. That breaks the failsafe logic in my conversion script.

Dovecot version dovecot-ee-2.1.16.3-1, x86_64, RHEL5. Dsync command used:

dsync -v -u username@example.net mirror maildir:/usr/local/atmail/users/u/s/username@example.net

With these dovecot.conf settings:

mail_home = /srv/mailstore/%256LRHu/%Ld/%Ln
mail_location = mdbox:~/mdbox

-jf