I have been studying this app from last month, have benchmarked it. Results are great! I love this software.
Thanks Kayako
Benchmark environment.
LAMP with eAccelerator and Zend Optimizer.
Email System used for receiving - Google Apps (Pop3 with SSL)
Email system used for sending - qmail (local to server)
Benchmark results.
Was able to receive around 500 mails per minute, it includes complex parsing rules including regex and filters.
Requirements at our side were quite complex in terms of customization but we found kayako to be very flexible, now all we need to develop is a Drupal-Kayako integration (which should be easy).
I would highely recommend to use Kayako, just be ready for some action with your dev team as we found kayako support to be not good when it comes to technical stuff.