How do I remove spam label from emails?

I am using mimemail api to sending emails, it works really great.
But now I stuck at the problem where emails sent by mimemail goes into the spam mail.

eg:

$sender    = 'senderemail@gmail.com';
$recipient = 'my_email_id@gmail.com'; // I checked this on gmail
$subject    = 'test subject';
$body       = 'test mail body';

mimemail($sender, $recipient, $subject, $body);

-- but this mail goes into the span listing on gmail.

Is there any solution to avoid this Spam label or any optional module?