Why does the From: line on mail received from LISTSERV say something like

From: My Test List on behalf of John Doe        

The actual mail was sent by John Doe.

This happens primarily when Microsoft Outlook is being used to read the mail, but it may happen with other mail user agents (MUAs) as well.  It is due to the way the MUA processes RFC822 "Sender:" headers in received mail.  The actual headers of the message you received were probably more like

Sender: My Test List <mytestlist@listserv.example.com>

From: John Doe <john@doe.com>

The intent of the Sender: field as used by LISTSERV is to show that the mail passed through an agent, which is perfectly legal according to the standard.  However, Outlook interprets this according to a different part of the standard, which implies that the Sender: field can also be used to indicate that some other person sent the mail on behalf of the person in the From: line (for instance, a secretary sending mail on behalf of an executive; it should be noted that RFC822 was written in the days when most executives didn't read or respond to their own email, if they even had email).

There is nothing wrong with either approach to the problem, but it can be confusing.  If it is desired to suppress this behavior, simply set

Sender= None

in the list header, and LISTSERV will not generate a Sender: field.