Need alternative to IMAIL Server

We currently use IMAIL and are not happy with their support and anti-spam feautures. Can someone recommend and alternative.
Our biggest problem is spam and how the server handles the spam. We also don’t have a full time mail administrator.

We thought of using Mailsite (Rockliffe) but we were told that their anti-spam features are weak.
Our biggest factors are:cost,anti-spam, easy of use and cost.

Thanks
Nando