A soft bounce is an email message that gets as far as the recipient's mail server (it recognizes the address) but is bounced back undelivered before it gets to the intended recipient. A soft bounce might occur because the recipient's mailbox is full, the server is down or swamped with messages, the message is too large or the user has abandoned the mailbox. We will attempt to deliver the email regularly for a few days. If it is still undelivered, it becomes a soft bounce.

A hard bounce is an email message that has been returned to the sender and is permanently undeliverable. Causes include invalid addresses (domain name doesn't exist, typos, changed address, etc.) or the email recipient's mail server has blocked your server. Servers will also interpret bounces differently, meaning a soft bounce on one server may be classified as a hard bounce on another.

We automatically move subscribers that hard bounce into a "Bounced Subscribers" category, so they don't receive future campaigns.

The different types of hard and soft bounces

Here's a full rundown of each of the types of bounces you might see in your Bounce Activity Report.

General Bounce (GB)

The email server could not deliver your email message, but the bounce message was unclear as to what kind of bounce it was. We treat these as soft bounces.
Example: "Subject: Undeliverable mail"

Soft Bounce - General (SB)

The email server is temporarily unable to deliver your message to the recipient email address.
Example: "Connection timed out."

Soft Bounce - Dns Failure (SBDF)

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 (SBMF)

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"

Soft Bounce - Message Size Too Large (SBMS)

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 (TB)

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 - Attachment Detected (MBAD)

Indicates that the recipient's email server is blocking your email because it contains an attachment.
Example: "552 Disapproved attachment"

Mail Block - Relay Denied (MBRD)

Indicates that the recipient's email server is blocking email from our email server.
Example: "551 relaying denied"

Non Bounce (NB)

We determined that the message was not a bounce. This could be a recipient reply, or maybe a bounce format that we didn't recognize.

Mail Block - General (MB)

Indicates that the recipient's email server is blocking email from our email server.
Example: "550 Message REFUSED by peer"

Mail Block - Known Spammer (MBKS)

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 (MBSD)

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"