Under the hood it's all Cyrus IMAPd server with a ton of customisations (seriously, it's a lot - it's kind of getting out of hand - I need to tidy some up and push them back upstream)

The code we run in production is all available to anyone who wants to play with it, though the usual caveats about "it's not particularly well tested for any configurations other than exactly the one we run" apply.

https://github.com/brong/cyrus-imapd/branches/fastmail

brong@prin:~/src/cyrus-imapd$ git log --oneline cmu/master..HEAD | wc -l 632

brong@prin:~/src/cyrus-imapd$ git diff cmu/master HEAD | diffstat | tail -n 1 149 files changed, 39060 insertions(+), 4314 deletions(-)