Why is my recipient on the suppression list?

Overview

The suppression list (Settings > Suppression List) is a list of email addresses that are permanently excluded from your email lists and will not receive future emails. The most common way a recipient is added to the suppression list is because of a hard bounce. 

Hard Bounce
A hard bounce is an email message that has been returned to the sender because the recipient's address is invalid. A hard bounce might occur because the domain name doesn't exist or because the recipient is unknown.

Other recipients can be added to the suppression list using these methods: 

Spam complaint
Subscribers that flag an email as spam are immediately added to the suppression list.

Manually Added
At times a partner may decide to manually add recipients to the suppression list using Settings > Suppression List. 

Unsubscribe
If the partner has enabled the unsubscribe option Add Any Unsubscribes to Global Suppression List in Settings > Company Profile, unsubscribed contacts will appear in the suppression list for all contact lists to which they belong.

Soft bounce
A soft bounce is an email message that makes it to the recipient's mail server, but is bounced back as undelivered before it gets to the intended recipient. This might happen because the recipient's inbox is full and it may be deliverable at a later time. After three delivery attempts, it will become a hard bounce. Here are some examples of soft bounces:

• Transient bounce
  A transient bounce is often generated by the sender's email server; this indicates that a message could not be delivered, but that the server is still trying to do so. Usually, a transient bounce can be safely ignored. You might receive this message: Warning: message still undelivered after 4 hours. Will keep trying until message is 2 days old.
• Address Change
  An Address Change response means a recipient has changed their address and is sending an automatic reply to notify senders of their new address.
• Auto-Reply
  Auto-Replies (generally in the form of an out-of-office notice) are usually sent by a recipient's email client. Unlike bounces, these indicate that an email recipient is temporarily unavailable. These notifications are useful when sending time-sensitive information to recipients, as they serve to alert you that the recipient may not see it until later. These can otherwise be safely ignored.
• Challenge/Response
  A Challenge/Response reply is a message sent by special filtering software installed by the recipient designed to accept messages from only senders they know. This type of filter automatically sends a reply with a challenge (a question or required action) to the sender of the email. If the challenge is not completed correctly, the message is not delivered.
• DNS failure
  The email server is temporarily unable to deliver your message to an email address because of a DNS problem.
• Email blocked
  Indicates that the recipient's email server is blocking email from your email server. You may see the following messages returned to you:
• - 550 Message REFUSED by peer
• - 552 Blocked by filters
• Mailbox is full
  The email server is temporarily unable to deliver your message to the recipient email address because the recipient's email box is full.