What is the supported variable type in the Output property field of all Get Mail activities?

In the context of Get Mail activities, the Output property field is designed to capture the data received from email messages processed by the activity. The correct answer is a List of MailMessage because this variable type is specifically tailored to handle multiple email messages efficiently. When you retrieve emails, it's common for tasks to require the ability to process several emails at once, hence the list structure that allows for multiple items. Each email message retrieved is represented as a MailMessage object. This design facilitates subsequent operations like iterating over the emails for processing actions or extracting specific information.

Other variable types like String, Integer, and Boolean are not suitable for storing multiple email messages. A String can only hold text data, an Integer is limited to numerical values, and a Boolean can only represent true or false conditions. Consequently, these types would not accommodate the complexities and requirements of handling multiple email messages returned from such activities.