Mails generated in PHP going to Spam (in Gmail only)

I am using Xenforo and the following script to send mail

<mail:subject>    {{ phrase('user_email_confirmation_subject', {'boardTitle': $xf.options.boardTitle    }) }}</mail:subject>{{ phrase('user_email_confirmation_body_html', {'username': $user.username,'board': '<a href="' . link('canonical:index') . '">' . $xf.options.boardTitle . '</a>'}) }}<p><a href="{{ link('canonical:account-confirmation/email', $user, {'c': $confirmation.confirmation_key}) }}" class="button">{{ phrase('confirm_your_email') }}</a></p>

When I use this script, the mail is going to spam for Gmail only.

How can I prevent email going to spam?