Suspicous SMTP attack

[Jacky So <jso @ hknet . com>]

Hi,

Recently, we are encountered non-stop incoming SMTP packet to our mail
Lan based SMTP gateway for more than a week. This suspicous access
coming from the same Mail host which are one of Mail Hubs of a large
ISP. Fortunately, this attack was blocked by firewall and being
reported. But, we haven't any idea and information of the attacking
source. 

I suspect those are email bomb
Please advise how and what we can deal with this kind of suspicious
attack. 

Cheers,
Jacky So