I've had some discussion with the developers of the Open-XChange
Groupware server about using MySQL as a storage backend.
As it turned out, a long-standing bug report has just recently been picked up and the first patches have been posted.
The procedure on how to get OX working with MySQL is also described on a Wiki page and there is a discussion about this in their Forum. So if you're interested in testing Open-XChange with MySQL, give this a try and provide them with feedback! Thanks for your support.