In the Output property of all Get Mail activities, the supported variable type is List<MailMessage>. The Output property is the property that stores the result of the activity in a variable. The Get Mail activities are the activities that retrieve email messages from various mail servers, such as POP3, IMAP, Outlook, or Exchange. The result of the Get Mail activities is a collection of email messages that match the specified criteria, such as the mail folder, the filter, or the top count. The collection of email messages is represented by the List<MailMessage> type, which is a generic list that contains MailMessage objects. A MailMessage object is a class that represents an email message, with properties such as Subject, Body, Attachments, From, To, etc1. For example, if the Get Mail activity is used to retrieve the email messages from the Inbox folder of an Outlook account, the Output property can be configured with a variable of type List<MailMessage>, such as mailList. The mailList variable will store the email messages as a list of MailMessage objects, which can then be accessed or manipulated using other activities, such as For Each, Save Attachments, or Send Mail2.
References: MailMessage Class and Get Outlook Mail Messages from UiPath documentation