imap_open will not open a stream if your server operates with Transport Layer Security (i.e. TLS) imap_open connects with SSL if its there. So try opening mailbox as
$mailbox="{mail.domain.com:143/imap/notls}";
or
$mailbox="{mail.domain.com:110/pop3/notls}"; This works...
Some mail server requires you to provide username@domain.com so you can always use. user@doamin.com
$conn=imap_open($mailbox, $username, $password);
Some server may ask for username as "user=user@domain.com"
:)