Up until about 20 minutes ago I was unaware you could specify multiple recipients in a mailto tag:
<a href='mailto:firstname.lastname@example.org,email@example.com'>Email Us</a>
(Tested in Firefox, IE6, Outlook 2003 and Thunderbird on Windows)
Activating this link will open up the users default email client and pre-populate the to: fields with the multiple addresses.
But (isn’t there always a but?), there is an upper limit on the number of people you can address.
I’m not sure what the upper limit is, but 539 (the current size of my test data) is too much. Activating the link does nothing.
I’m not sure if this is a problem with the mailto: tag, the browser or the email client.
More info as I explore further…
(unconfirmed) Looks like there might be a maximum length for a href attribute of around 250 characters)