As documented you have to double the ampersands in a template form if
you actually want an ampersand to appear.  If you don't then LISTSERV
thinks you are talking about a LISTSERV substitution variable.  Try

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On Wed, 1 Nov 2000 14:25:16 -0800 John, Laura said:
>In the bottom_banner section of the .mailtpl file, I put the following text:
>
>>>> BOTTOM_BANNER
>
>=========================================
>
>To remove yourself from this list, simply click this link:
>mailto:[log in to unmask]&body=signoff%20afterbell and send the email.
>
>However in the Newsletter, the mailto link ended up (notice the semi-colon
>after the word BODY):
>
>=========================================
>
>To remove yourself from this list, simply click this link:
>mailto:[log in to unmask]&BODY;=signoff%20afterbell and send the
>email.
>
>Is there any way to avoid this?
>
>Thanks,
>Laura John
>Director of Program Management
>MSNBC.com