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I get the following error each time I try to use this add-on now. It stopped working about 3 or 4 months ago. When I send a sleuth request, the email is returned from my email provider's server with the error shown.
<trace-request@mailsleuthonline.com>: Host or domain name not found. Name service error for name=mail.mailsleuthonline.com type=A: Host found but no
data record of requested type

So it appears the domain for mailsleuth has become blocked or unobtainable.