Today, while having a look at a Zimbra server running z-push for ActiveSync, I found these errors in log:

03/05/2017 03:18:36 [ 2372] [ERROR] [info] Zimbra->SoapRequest(): SOAP FAULT: Error Code Returned [mail.TOO_MANY_CONTACTS]
03/05/2017 03:18:36 [ 2372] [ERROR] [info] Zimbra->SoapRequest(): SOAP FAULT: Error Code Returned [mail.TOO_MANY_CONTACTS]

Z-Push was kind of spamming this message many times a minute in z-push-error.log.

The answer is pretty easy: by default Zimbra allows at most 10000 contacts per mailbox.

This setting is stored into the Class of Service (CoS), and it’s easy to fix.

On the Zimbra Admin WebUI: go to Configure > Class of Service > default (or your custom one) > Advanced > Available spaces. There you can find the setting Max contacts per folder.

From Zimbra CLI: just run

zmprov mc default zimbraContactMaxNumEntries 100000