Outlook does not support mailto links by default
I didn't know that
Outlook does not support the RFC 2822 standard by default. So a standard compliant mailto link that has the addresses separated by comma's simply doesn't work with Outlook. If however the addresses are separated by semicolons it does. Unfortunately Mozilla's Thunderbird doesn't work with semicolons. What to do, what to do?
I would ask my clients that use MS Outlook to navigate to Tools -> Options -> E-mail Options -> Advanced E-mail Options and enable 'Allow comma as address separator' if it was not for MS Outlook to simply ignore this setting.
Cheers,
Wesley