Personally I have more questions than answers.
- What size are the messages?
- Do we know which commands are slow? Specifically is it LIST or RETR? There are some optimisation we can do around LIST.
- How many messages are in the mailbox when we execute LIST?
- What setting do you have for the mail store? Is compression enabled? What is the page size set to?
It would be interesting to compare the performance on MySQL to PostgreSQL. The paged store implemenation won't be the most performant, however reads should still be quicker that write. We may have to look at doing some profiling at some point.
I set up your account on set. Give it a try. Message size doesn't seme to matter. LIST seems fine. RETR is the problem. I have the installer defaults (which I took from the dev jboss-service.xml).