I'm looking for the fastest mail server software currently available. I'm looking to store around 900GB of mail. I need to be able to perform full body searches across all messages via IMAP and get a response within a reasonable time. Currently I am using Cyrus and I seem to be getting at its very limit even with enabling the squatter index.
I have looked at Zimbra and it ticks every box part from the fact it does not support per user flags across public folders.