"To" field in mailings

My mailing list receipients have been telling me that when viewed in Outlook, the messages I send display “Test from XXXX” in Outlook’s TO field instead of “Mailing List @ (mydomain).com” which is what it used to say. This is confirmed when viewing the message header. When editing the Mailing List there is no field where the words “test” appear. Where is this coming from, and how do I get rid of it?