First of all, please check which email status the contact has: temporarily blocked, blocked, spam or unsubscribed?
A temporary block is lifted after a certain period of time. If the email cannot be delivered the first time, Quentn will unblock the email address after 72 hours. Beware, however, because the time span increases with each additional block (up to 75 days)! After five temporary blocks, the email address is permanently blocked.
If the contact has unsubscribed, they have actively done so and do not want any more messages from you. If they have unsubscribed by mistake, they can simply re-register with you (e.g. in a form or similar) and perform the double opt-in process again to get a "Confirmed" status again.
If the email status was marked as spam, the contact themselves or their email client has classified your email as spam and rejected it. If your emails end up in the recipient's spam folder, it is helpful if they move the emails to their main inbox and add you as a contact. This way, their email client learns that your emails are not malicious.
If a contact is permanently blocked, ask them to register with a different email address.
Reasons for blocking
If you want to know why a contact has been blocked, we recommend that you look at the error message. To do so, click on the email status to read the "Full server message". Here you can find out the reason for the block.