User-customizable forms in an electronic mail system enable an administrator to select customized field types and customized behaviors. The electronic mail system provides for the receiving and displaying of mail information in conjunction with the use of a user-customizable form. An administrator creates a customized field of a customized form by selecting a field type from a among multiple defined field types and a behavior from among multiple behaviors. When data is entered by a user to the customized field having the selected field type, the selected behavior is applied to the data. In this manner, the administrator has the flexibility of creating a variety of customized forms in the electronic mail system, and is not limited to a relatively small number of predefined forms.