How did you take the backup?
While taking backup, you need to preserve the time stamp.
and while restoring, you need to do the same.
For example.
Mail dir = /home/xyz/mail
for backup: tar -czpf xyz-mail.tgz /home/xyz/mail
for restore: tar -xzpf xyz-mail.tgz
Here, the option -p is used to preserve the timestamp and ownership.