Zimbra activities cause high traffic usage??

[nameless]

hi, i'm very new in Zimbra.

I just want ask if anyone can explain to me what activities are done by zimbra server (multi servers installation) when at night.

It is because my customer complained to me that their email filter appliance detect that zimbras server do such activities that cause high traffic usage. (start from 7pm till 6am everyday). Anybony know why?

thanks,

[nameless]

any comment?