This tool which seems to be in beta for now lets you test your exchange services externally

These tests are available as of today:

• Microsoft Office Outlook 2007 Autodiscover Connectivity Test
• This test will walk through the steps Microsoft Office Outlook 2007 uses to connect to Autodiscover
• Microsoft Office Outlook 2003 RPC/HTTP Connectivity Test
• This test will walk through the steps Microsoft Office Outlook 2003 uses to connect via RPC/HTTP
• Microsoft Exchange ActiveSync Autodiscover Test
• This test will walk through the steps a Windows Mobile 6.1 device (or another AirSync licensed device) uses to connect to the Autodiscover Service
• Microsoft Exchange ActiveSync Test
• This test will simulate the steps a mobile device uses to connect to an Exchange Server using Exchange ActiveSync.
• Inbound SMTP Email Test
• This test will walk through the steps an Internet e-mail server uses to send inbound SMTP email to your domain

You can find the test site here and the domain is owned by Microsoft but as always when sending passwords to unknown sites use a dummy account