Thanks very much for your advice. But i think my problem is a little
different from yours. The email server is remote server and this is the
only server. And there is no extra harddisk in the server. And the
network between my backup server and the email server is not so good.

And large data is not suitable to transfer between the backup server
and the email server. No extra harddisk, so i could not rsync the mail
spool from the working harddisk to the extra harddisk.

Is there any incremental backup method suitable for my situation?

Best regards

Wang Penghui