as far as I know, the ctime of a file can not be set "manually". It is alwaysset to the date when the file is created.
I did not had this problem yet so I dont know if there is a different way to fix the issue in courier without setting ctime.
For future backups, you might want to check if your backup program is able to store all file permissions and dates like tar does it with:
tar -pcfz archive.tar.gz /var/vmail