Thanks Mike, I appreciate your follow up here. The DNS/Autodiscover suggestions Tony has passed along are for good reason at the very least :) Outlook these days uses a Microsoft-based discovery system to find account details and facilitate the connectivity, so the autodiscover deployment must be reachable from the outside in order for Outlook account set up to complete successfully.
Because of this, differences between internal/external DNS can introduce issues and this is likely what he's looking at now. We do see issues like these occasionally and in most cases there is an internal DNS record pointing to an IP which isn't bound to the hostname in IIS, the SSL certificate for that hostname doesn't support being called on that IP, etc. When fully functioning both internal and external machines should be able to:
- Reach the SRV record you have set up in DNS which points to mail.customer-domain.com.
Additionally, the requirements above need to be specific to the domain you're setting up a MAPI account for. So if the user account is customer-domain.com and you have everything pointing to mail.server.com this too can introduce issues due to how Microsoft is handling the account set up procedure.
Based on your follow up though it sounds like these are checking out during your troubleshooting and we likely need to dig into the Fiddler traces further to see whats going on. Did you get a ticket submitted on this one already? If so please ping me with the ticket number and I can add some notes here as well.