Currently I am using vtiger 6.5 and i have same issue. I solve this by changing following seetings in php.ini 1)enable php_imap extension 2)later on check PHP_SAPI and update this in Vtigercron.php which is in yourvtigerdir/vtigercron.php here you see this line 42
in this line you need to change PHP_SAPI which is your sapi according to your php interface.3)You can check your sapi by echo PHP_SAPI; or echo '<pre>',print_r(php_sapi_name()),exit;
4)After above changes, Change
a) select organization type :: other
b) Male server ip :: (enter your ip or mailser details)
c) user name :: user or this mail as your personal mailid
d) password :: enter your own password.
e) protocol :: IMAP2 or IMAP4 (check by changing protocol)
f) SSL options :: At starting level Use NO TLS.
g) Certification validation :: Validate cert.
f) referesh time :: 5