I'm currently coding an HTML email, using MailChimp to test it, and eventually to send it.
The email advertises several sales, which are displayed using the HTML
<strike>$12.95</strike> SALE! $9.95
$12.95 SALE! $9.95
An in-line style of
text-decoration:line-through is your best option and it seems to be widely supported. The PDF on that page has a much larger result set for many more clients.
Also, it might be worth trying out fractal as they claim to be able to optimise and validate HTML e-mails. I have not used it myself. Caveat is that it looks like you need to sign up to the service.