We've recently upgraded to 5.0.2_GA_1975 RHEL4_64 NETWORK edition

I'm testing this with Pidgin 2.4.0. XMPP Requiring SSL/TLS, Port 5222, no proxy.

The Problem:
3rd party IM clients work when a domain is using internal authentication, however they fail to connect when using active directory authentication. I can't find any logs pertaining to this server side, and pidgin logs are:

Code:
(09:38:57) account: Connecting to account user@zimbra.domain.com/Home
(09:38:57) connection: Connecting. gc = 03B32838
(09:38:57) dnsquery: Performing DNS lookup for zimbra.domain.com
(09:38:57) dnsquery: IP resolved for zimbra.domain.com
(09:38:57) proxy: Attempting connection to 172.25.1.10
(09:38:57) proxy: Connecting to zimbra.domain.com:5222 with no proxy
(09:38:57) proxy: Connection in progress
(09:38:58) proxy: Connected to zimbra.domain.com:5222.
(09:38:58) proxy: Error connecting to zimbra.domain.com:5222 (Unknown error).
(09:38:58) proxy: Connection attempt failed: Unknown error
(09:38:58) account: Disconnecting account 03AB60F0
(09:38:58) connection: Disconnecting connection 03B32838
(09:38:58) connection: Destroying connection 03B32838
Any ideas/help much appreciated!