my mac clients having upgraded to 10.10.3 are facing major issues - especially the upgrade to 10.10.3.

This Apple Support Community confirms those issues (for other mail server products):

What we've found out so far:
- The larger the mail box, the worse the issues
- Causing high load on the server side (even zarafa-gateway crashes sometimes (after changing to pthread mode):
In the end - currently recommending to use thunderbird on OSX 10.10.

Seems to be related to SSL performance issues and OSX constantly caching folders and emails.

Any ideas, what can be done on OSX mail config? Turning off SSL / IDLE command doesn't seem to cure the issue.

Best regards