public override Hashtable PstFields { get; }

Public Overrides ReadOnly Property PstFields As Hashtable
	Get

Property Value

The below are PST fields keys for Activity items. Each key represents a particular property of this PST item. Most values are optional so that you need to check if the particular key exists with ContainsKey(Object) method (Hashtable.ContainsKey).

When you get this PST object as MailMessage using GetAsMailMessage method, these fields will be added to the resulting message as "X-Activity-" + PstFieldName headers. Example is "X-Activity-LogType". Each field value will be converted into a string.

Reference