When I was testing imap_headerinfo() with an e-mail that had multiple recipients (multiple e-mails in to to: and/or cc: field), I noticed that imap_headerinfo() was failing hard for me on PHP 5.2.10-2ubuntu6.4 with Suhosin-Patch 0.9.7 (cli).

Rather than providing me an array with each and every e-mail address listed in the to and/or cc fields, it was only providing me the first listed.  This was disappointing.

   [to] => Array
        (   
            [0] => stdClass Object
                (   
                    [mailbox] => game
                    [host] => blunts.com
                )
        )

Luckily, there was a cool workaround to this problem:

imap_rfc822_parse_headers(imap_fetchheader( string ); which subsequentally worked like a nice little pet would:

   [to] => Array
        (   
            [0] => stdClass Object
                (   
                    [mailbox] => game
                    [host] => blunts.com
                )
            [1] => stdClass Object
                (   
                    [mailbox] => dutch
                    [host] => masters.com
                )
        )

TL;DR: 
So in other words, instead of imap_headerinfo() use imap_rfc822_parse_headers(imap_fetchheader()).

Hope this helps anyone else that runs into this issue from now into the future.  This post was suggested by people in #PHP on FreeNode IRC.