What email clients should I test my emails against?

We recommend thorough testing in a range of email programs and web mail sites. Generally, the top four below cover a high percentage of most mailing lists though bear in mind a B2B list will likely have a lower percentage of Hotmail users than a B2C one for example.

Top email clients for testing on:

Outlook 2007
Hotmail / Windows Live Mail
Google GMail
Outlook 2010
Thunderbird
Yahoo! Mail
Windows Mail / Outlook Express / Outlook 2003 (largely the same)
Roundcube web mail
AOL web mail
Tiscali web mail