I had one very interesting Issue with Exchange 2013 server and MAPI over HTTP.
When creating profile internal Outlook is failing on the last step “logon to server” with the well known error:
The connection to Microsoft Exchange is unavailable. Outlook must be online or connected to complete this action.
When turned off MAPIoverHttp for the user, he connects without problem. In logs we see 401 unauthorized from mapi/nspi .
After testing with different internal URL vor MAPI Virtual directory. I found that the problem exists only with the name which was currently assigned as internal and external URL for mapi.
It was Remote.domain.com/mapi
It turns out that there is a Computer object in AD with name Remote. This is causing outlook to make connection attempts to this PC.
setspn -l remote
shows that there is HTTP spn for this record.
So the solution is to use another DNS entry for internal MAPI URL or remove the computer object.