BM 3.9 SP2

I have users attempting to access a remote Ironport secure email server. They are able to login to user accounts and see their email list. However, when they attempt to open email they immediately get returned to the login page. During testing, when the proxy server is bypassed, we DO NOT have this problem.

I have tried the following with no success:

Disabled Enforce Access Rules
Added the remote domain to Cachable Object Control