Password Reset emails being marked as spam
A couple of questions


What CMS are you using?
How do you have your outbound email set up?
Are other emails getting marked as spam?
When you view source of the email, are there any headers inserted by the receiving mail server that let you know why it was marked as spam?


Password reset emails are typically sent by the CMS, not CiviCRM and you will need to make appropriate changes there. Both Drupal (smtp) and WordPress (WP Mail SMTP) have SMTP plugins/modules that allow you to use a 3rd party SMTP server to send email.