EAS with Exchange 2007

[pj_teeter]

ISA 2006 fully patched, running on Windows Server 2003 R2 Enterprise SP2.
Exchange 2007 SP2 RU 4 on Windows Server 2008 Enterprise SP2.

Have successfully published OWA policy using forms-based authentication. Am
able to connect via externally mapped IP/hostname successfully from IE,
Safari, etc.

Attempts to verify connection to Exchange Activesync URL via published ISA
policy continue to fail.

Am still able to obtain 501/505 error code with IE, Safari, etc. when
connecting to EAS URL
(https://external.exch07.hostname/Microsoft-Server-Activesync &
https://internal.exch07.hostname/Microsoft-Server-Activesync) which I'm led
to believe verifies functionality.

However when trying to connect via
https://external.isa2006.hostname/Microsoft-Server-Activesync, can
authenticate at ISA but once authentication seems to work obtain 500 error
code - target principal name is incorrect.

Have tried changing external URL for EAS to variety of values - no change in
behavior & no success.

I can burn a support case but figured I'd try the forum first. Anyone have
any ideas?

Thanks.