Most customers have a need to modify their Exchange display templates so they can control what Outlook users see when they are looking at the Offline Address List data. To modify an existing template you can follow this blog: http://blogs.msdn.com/dgoldman/archive/2007/02/14/how-to-add-edit-change-and-revert-address-and-details-templates.aspx
Now more often than not the Active Directory attributes will not exist for you to make a change. Let me give you an example.
Your company sets up a new IP Phone system and you would like to modify the display templates so you can add this new IP Telephone Address to your Outlook Display Template. In order to do this you need to add a custom mAPIID property.
Now what is a mAPIID? The MAPI-ID Attribute is an integer by which MAPI clients identify a particular attribute in the active directory.
NOTE - What is not supported!!
1. The reason why we do not offer the steps on how to do this is because it has not been tested and therefore is not supported on any version of Microsoft Exchange.
2. Trying to add custom mAPIID's causes schema modifications which can leave your system in an unstable state. These changes can and often do make more problem that what it is worth.
3. Installing another software applications might modify the schema and may change your mAPIID value back to its original value thus breaking what you setup.
4. We also do not support re-using existing MAPI IDs for other attributes, therefore Microsoft development *will not* fix any issues or bugs that may arise from you making this type of change.
5. This is the Exchange Product Groups official stance!!
NOTE - What is supported!!
1. The official recommendation is to use one of the 15 custom Exchange extension attributes that have been added to the interface for you to use 🙂