You may look into imap_search
$sentfolder = "{".$host."}".'SENT'; //sent folder name
$outbox = imap_open($sentfolder,$username ,$password) or die('Cannot connect to Mail: ' . imap_last_error());
$sent_msgs = imap_search($outbox, 'ALL');
OR:
// match messages with the \\ANSWERED flag set
$sent_msgs = imap_search($outbox, 'ANSWERED');