Swift_Mime_Headers_AbstractHeader | --Swift_Mime_Headers_IdentificationHeader
Located in File: /vendors/swiftMailer/classes/Swift/Mime/Headers/IdentificationHeader.php
This is not necessarily RFC 2822 compliant since folding white space will not be added at this stage (see toString() for that).
This method returns an array of IDs
If multiple IDs are set only the first is returned.
This method takes a string ID, or an array of IDs