Ampersand HTML entity messed up

My module is producing a link to include in the email being sent.
<a href="http://local.mysite.org/event/5423?instance=5421&amp;launch=true">Participate</a>

This looks to be correctly formed HTML (it is built with the "l" function). But when the email is sent, it is rendered with an extraneous semi-colon after the ampersand:
<a href="http://local.mysite.org/event/5423?instance=5421&;launch=true">Participate</a>

This is preventing our users from connecting to the event.

If the link is directly in the email body, it seems to be correct, but when I produce the link with either Views or Tokens code, it is screwed up like this. If I disable Mimemail, the result looks correct, although it is not clickable.