Recently I ran into a rather annoying issue and after lots of testing and research I think I found the reason. Here's the scenario: Windows SharePoint Services 3 site, internal access http://wss3, external https://share.domain.com:444. Central Admin Alternate Access Mapping had the default zone http://wss3 and the intranet sone https://share.domain.com:444. The site works great from inside and outside the network. Outlook 2010 installed on laptops.

The Outlook users browse to http://wss3 internally then connect their Outlook to a calendar. The calendar would download the first time and then no updates could be sent or received. Outlook Send/Receive errored

Task 'SharePoint' reported error (0x0004060E) : 'Outlook cannot connect to the SharePoint List (Team Site - Calendar). Contact the SharePoint site administrator.  HTTP 502. An error occurred connecting to http://wss3. A connection to https://share.domain.com:444 , an alternate Web address for this SharePoint site, will be attempted.

They can browse to http://wss3 all day long, but come to find out the internal DNS doesn't know about the external address. I didn't think it was necessary.

After much testing by deleting the calendar from Outlook, changing the Alternate Access Mapping and DNS, it appears SharePoint tells Outlook to use the Intranet zone when making the link. This is great because the laptop users leave the network and still want access to SharePoint calendar. The issue is that when they're internal it is trying to use that address as well. We corrected the internal DNS so the external address would resolve locally and everything works great now.