Wow.. and I thought _I_ was using an old version (0.99.14)... Seriously, update now. Between just 0.99.10 and 0.99.14 there was a considerable number of important fixes. And between 0.99 and 1.0beta1, there's reportedly large performance increases, improved indexing, improved mbox handling, and a large number of added features.
I'm hoping your reason for using such an old version is that's what your distro has packaged.
My reason for using such an old version is that up to the recent report of mail not appearing there were no problems whatsoever and it is running in a protected environment - so I stuck to "never change a runnning system". I have other machines running newer versions, but could never make out a difference in stability. Performance might not be such an issue in small environments with <15 users. As for the 1.0 jump, I am waiting for the noise on this list to settle before upgrading.
Yours, Jakob Curdes