From Encode::MIME::Header

This module implements RFC 2047 MIME encoding for an unstructured field body of the email header. It can also be used for RFC 822 'text' token. However, it cannot be used directly for the whole header with the field name or for the structured header fields like From, To, Cc, Message-Id, etc... 

WebGUI::Mail::Send encodes these headers on newer perl (5.24) atleast.