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.