When an email cannot be delivered to a recipient's inbox it is returned as either a soft bounce or a hard bounce For soft bounces there are multiple reasons why the email could not be delivered and these are displayed in your Bounce Summary report, as shown below.
Soft Bounce Reasons
Here is the full list of soft bounce reasons you might see in your campaign report:
The email server could not deliver your email message, but the bounce processing tool could not determine a specific reason for the bounce. Normally that is because the bounce message from the recipient's server was very broad. We treat these as soft bounces.
Example: "Subject: Undeliverable mail"
Soft Bounce - General: The email server is temporarily unable to deliver your message to the recipient email address.
"Connection timed out."
Soft Bounce - DNS Failure: The email server is temporarily unable to deliver your message to the recipient email address because of a DNS problem.
Example: "Host is unreachable"
Soft Bounce - Mailbox Full: The email server is temporarily unable to deliver your message to the recipient email address because the recipient's email box is full.
Example: "Mailbox over quota"
The email server could not deliver your message to the recipient because the message size is too large.
Example: "Exceeded maximum inbound message size"
Transient Bounce - The email server temporarily can not deliver your message, but it is still trying.
Example: "Warning: message still undelivered after 4 hours. We will keep trying until message is 2 days old"
Mail Block - Relay Denied: Indicates that the recipient's email server is blocking email from our email server.
Example: "551 relaying denied"
Mail Block - General: Indicates that the recipient's email server is blocking email from our email server.
Example: "550 Message REFUSED by peer"
Mail Block - Known Spammer: Indicates that the recipient's email server is blocking your email because it believes you are a spammer.
Example: "REJECT Known SPAM source"
Mail Block - Spam Detected: Indicates that the recipient's email server is blocking your email because the message appears to have content that looks like spam.
Example: "550 Possible spam detected"
A hard bounce is a permanent delivery failure.
When you get a hard bounce it means the recipient's email address is invalid or no longer in use. Typically the domain name (the bit after the @) no longer exists or it no longer has registered mail servers. But it could also be invalid due to typos, for example: