Determining if my OpenBSD 3.3 box sent out mails

[Jim Lambert <JLambert@Futurex.com>]

Hello:

We had an issue at our company last night where our ISP detected a large
amount of emails coming from our IP address.  I want to make sure this
wasn't generated by my OpenBSD systems.  Is there something I can check,
some log or configuration, that will tell me if my system was the offending
system?  I have checked the maillogs and the messages logs in the /var/log
directory but didnt see anything overt. 

My systems are stock OpenBSD3.3 systems with DHCP, NAT, and PF enabled.  I
have also installed healthd which could email on a problem, but which isn't
configured to email on any problems.

Any information would be appreciated.

Jim