I'm afraid this won't really do what we want.  In general, when we send to two people at once, we make a *single* `To` field with what's called the `mailbox-list` format.  More or less, it looks like this:

    To: "Ricardo Signes" <rjbs@example.com>, "Bob Smith" <bob@example.com>

Although I *think* it is legal to have two To headers, it is not the done thing.  The problem that now arises is:  what *is* the expected input for this method?  As written, it expects a single string, meaning it would want you to provide the `mailbox-list` directly.  Fortunately, it should be safe to join two mailbox list tokens by concatenating with a comma-space.  (You may want to double-check [RFC2822](http://www.faqs.org/rfcs/rfc2822.html), but I think it's safe.

So, I think this should be:

    …(To => join q{, }, @_)…

Finally, you'll want to add tests **and run them**.  I was baffled to see the tests you'd written, because I knew they should not pass.  I ran them myself, finally, and they did not pass.  Did you run them, yourself, before submitting a pull request?