Incoming IMAP Hackers?

[chris at idlelion.net]

As I sat in my office this morning perusing the interwebs, my server 
started making a lot of noise. (It has very noisy hard drives.) I looked 
in the logs and saw repeated requests to connect to my IMAP server using 
guessed user names. All the authentications were failing, but it increased 
the load on the poor box and delayed its responsiveness to me.

It was fairly simple to manually add an IPtable rule to block this one IP 
address, and the attack stopped.

But I cannot think of a reason someone would want access to IMAP accounts. 
What purpose would this serve?

Chris