If you create user-defined fields and only use these fields within Outlook itself, then this article will be of little or no interest. If, however, you need to (or at some point, think you will need to) share information between Outlook and an external data source, this article will cover some points that can potentially save many problems – specifically in terms of how field names are constructed.

The terms <user-defined fields> and <custom fields> refer the same thing – fields defined by a user that apply to a contact in addition to the standard Outlook fields.

However, for ease of reference, the term <custom fields> is used for those user-defined fields that are defined as part of a custom form.

There are some essential differences between <custom fields> defined in a custom form and <user-defined fields> used with contact items using the standard Outlook form (messageclass – <IPM.Contact>) some of which are:

Every Outlook item is created with a <form> which translates to the message class name that is assigned to it. To state this another way, <out of the box>, a default form is used when a contact is created. In the case of contacts, this form is assigned the message class of <IPM.Contact>. It is the design of the form in use that dictates how information will be entered for a contact.