When enabling Incoming Email (Workspace) function "Allow an agent to submit work items for another person by email", FootPrints 12.1 will allow an agent to use sender= and contact= values when creating a new ticket by email only.
Warning:  Do not use contact=addressbookprimarykey to update existing tickets.

How to use sender=UserID in incoming email?

Using sender= with either "Email Address" or "User ID" will create the ticket as that user if there is a user account in Footprints with either that User ID or Email Address.  Using "sender=" will populate the "Created By" field in the container. In Footprints 12.1.05 and before the "Created By" user of the ticket will be linked as a contact if they also exist in the Address Book.  In Footprints 12.1.06 and above, the contact is not set by "Sender=" value. So 12.1.06 and above we need to use contact= to populate contact.
Please see these article about how incoming email are processed:
Incoming email processing
Creating tickets via email

How to use contact=AddressBookPrimaryKey in incoming email?

If contact= is used, the ticket will be created by the original submitter, in the "Created By" field, and the contact will be linked if a contact record is found in the address book that has a primary key value of that which was used in the contact= string.  The value you specify has to be the Primary Key in the Address Book to link the contact.  It also has to be a "User ID" or "Email Address" of a user in Footprints to show that the ticket was created by that user.

The main thing to emphasize is the Primary Key setting in the address book. The default is "Email Address", but most customers change to the "User ID" field.  To see this, edit the address book and contact item. Going into the "Fields" tab, note the last column "Primary Key" to note the field name set to "Yes".

To troubleshoot the incoming email processing of "sender=" and "contact=", enable to the Incoming Email (System) "Email Debugging", submit a test email, and review the Footprints.log per the article, How to troubleshoot Footprints 12.1 using the footprints.log?

This will show how the system tried to find a match for the value after "sender=" and "contact=".