[BUG][FIX] MailMessage with multiple to headers ( [er] MailMessage still needs more work )

[dway at mailcan.com]

This is not really the fault of Frank's changeset, but it seems that
MailMessage still needs more work.  If I create a MailMessage and use
addRecipient:, I can add a few To: addresses, which show up successfully
as separated by commas in the To: field of the MailMessage editor.  But
MailMessage still tries to wrap angle brackets around everything in the
To: field when it gets sent.  Like this:

<address1 at company.com, address2 at myco.com>

...which my SMTP server rejects.