Address Change (bounce_ac)
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-Replies (generally in the form of an out of office notification) are usually sent by a recipient's email client software. Unlike bounces, these indicate that an email is temporarily unavailable. These notifications are significant when sending time-sensitive information to recipients, as the recipient may not see it until a later time.
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 to the (alleged) sender of the e-mail. In this reply, the sender is asked to perform some action (like clicking a link) to ensure delivery of the original message. If the challenge is not completed, the message is not delivered.
DNS failure (bounce_df)
The email server is temporarily unable to deliver your message to the recipient’s email address because of a DNS problem.
Hard Bounce (bounce_hb)
A hard bounce is an e-mail 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.
Email blocked (bounce_mb)
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 (bounce_mf)
The email server is temporarily unable to deliver your message to the recipient email address because the recipient's email box is full.
Soft bounce (bounce_sb)
A soft bounce is an e-mail message that gets as far as the recipient's mail server but is bounced back undelivered before it gets to the intended recipient. A soft bounce might occur because the recipient's inbox is full. A soft bounce message may be deliverable at another time or may be forwarded manually by the network administrator in charge of redirecting mail on the recipient's domain. After three tries, it will become a hard bounce.
Transient bounce (bounce_tr)
A Transient Bounce is a message often generated by the sender's email server which indicates that a particular message could not be delivered but the server is still trying. Usually, a Transient Bounce can be safely ignored. You may receive the following reply:
Warning: message still undelivered after 4 hours. Will keep trying until message is 2 days old.