Do your recipients see your mailings as spam? Then they can indicate this via their e-mail client by means of a spam button.
This can lead to a poor sending reputation and that your mailings are blocked in advance. Make sure the unsubscribe link is clearly visible in your mailing. We recommend not only including an unsubscribe link in the footer, but also in the header. In addition, make sure that you always email to an up-to-date contact file with an opt-in for your mailings.