I did pretty big improvements with mail server recently, but have not touched plugins. I even don't know someone was using them.
It is really hard to give you any answer now, but if you are willing, you can send me the code of your plugin. I could install and check how it behaves. It will be easier for me to debug.
If you would like to share your plugin code, or at least, a simplified version of it, you can email it to me at archie at argosoft.com