I was introduced to Kerio MailServer from one of my customers. They like Kerio for their SMB because it is inexpensive and uses a flat-file database architecture which allows them to access objects down to the individual email. This makes backup, restore, mail merging, etc. very easy for them. It sounds like it really meets their needs nicely.
They recently were looking for some added data protection for Kerio disaster recovery. While restoring Kerio from backup tapes is easy enough, they want to provide a better recovery point objective (RPO) and Recovery Time Objective (RTO). They found that SteelEye DataKeeper fit the bill perfectly as they were able to do real-time replication and have a standby Kerio server up and operational in just a few minutes.
Here is a blog post that discusses their experience during their evaluation of SteelEye DataKeeper.