When an email address or domain has been determined to be unreachable after a single attempt, the message is considered a "hard bounce".
Why this is important
There are many types of different bounce notifications such as hard-bounces, soft-bounces, email blocked, etc. Understanding each of these types will help you identify the best way to deal with them and properly take the best course of action for the health of your lists.
Types of bounces
- Soft bounce (bounce_sb)
When an email message reaches the recipient's mail server but is returned as undelivered, it is considered a soft bounce. In some cases, the recipient's inbox might be full, so it may be delivered later. Three delivery attempts will result in a hard bounce.
- Transient bounce (bounce_tr)
Often, the sender's email server generates a transient bounce, informing the sender the message wasn't delivered but the server is trying again; this will continue until two days have passed since the message was sent. However, there is usually no need to worry about transient bounces. Also, you might receive the following message: Warning: message still undelivered after 4 hours.
- Address Change (bounce_ac)
An Address Change response indicates that the recipient has changed their address and is notifying senders of the change.
- AutoReply (bounce_ar)
The recipient's email client usually sends an auto-reply (usually in the form of an out of office notice). These indicate that an email recipient is temporarily unavailable, unlike bounces. These notifications are useful when sending time-sensitive information, as they alert you that the recipient may not see it until later. Otherwise, they can be safely ignored.
- Challenge/Response (bounce_cr)
A Challenge/Response reply is a message sent by special filtering software that only allows messages from known senders to be accepted. This type of filter automatically sends a reply with a challenge (a question or action required) to the sender of the email. The message will not be delivered if the challenge is not completed correctly.
- DNS failure (bounce_df)
This indicates that a DNS problem has temporarily prevented email delivery to an address.
- Hard Bounce (bounce_hb)
A hard bounce is an email message returned to the sender due to an invalid recipient address. The domain name might not exist or the recipient may not be known, resulting in a hard bounce.
- Email blocked (bounce_mb)
This indicates that the recipient's email server is blocking your email. You may see the following messages:
– 550 Message REFUSED by peer
– 552 Blocked by filters
- Mailbox is full (bounce_fm)
A full recipient email box is preventing the email server from delivering the message to the recipient.