Mailbox permissions for user@domain.com are broken and can't be fixed

Introduction

If mail files were manually moved or modified on a server, this can lead to the group and ownership being incorrect. This will cause mail to not be imported into the webmail interfaces. 

Procedure

Be sure to replace user with the actual username.

First, confirm the files and groups are owned as root. 

find /home/user/mail/ -group root -user root

./domain.tld/usr/cur/1482076549.H333078P21227.host.servername.com,S=5694:2,S
./domain.tld/usr/cur/1471208254.H290545P10450.host.servername.com,S=3379855:2,S
./domain.tld/usr2/cur/1398635418.H332872P84537.host.servername.com,S=1132377:2,S
./domain.tld/usr2/cur/1338608749.H957451P24697.host.servername.com,S=1571:2,RSa
./domain.ctldm/usr3/cur/1338858170.H14687P13660.host.servername.com,S=2047:2,S
./domain.tld/usr/cur3/1338868177.H467635P2592.host.servername.com,S=10567:2,S
./domain.tld/usr/cur3/1338868920.H192113P8811.host.servername.com,S=19808:2,S
./domain.tld/usr/cur4/1339281746.H118421P31249.host.servername.com,S=19618:2,S
./domain.tld/usr/cur4/1339644063.H280841P10781.host.servername.com,S=3778:2,S
./domain.com/usr/cur4/1339689314.H242260P17541.host.servername.com,S=85515:2,S

First, as root, login to the server and run

cp -Rav /home/user/mail{,.temp-backup}

Second, run:

find /home/user/mail -group root -user root -exec chown -v user:user {} \;

That will find and own the files as root, then change them to user:user