How are you sending e-mails? e.g. sent via an e-mail client, web mail or via a system/web script that's running on your hosting?

If you are sending a large amount of mail (i.e. a newsletter), it could be that the e-mail doesn't conform to the bulk-sender guidelines, for more info see: https://support.google.com/mail/answer/81126?hl=en

Otherwise, it could be the content of the e-mail which is being considered suspicious and being mistaken for spam.