Hard vs soft bounce: which failed emails to stop sending

How it works

When a message can't be delivered, the receiving mail server sends back a reason. Your sending provider reads it and sorts the bounce. Resend labels bounces Permanent (hard) or Transient (soft). Postmark uses types such as HardBounce, SoftBounce and Transient.

• Hard bounce. The address is unknown, or the domain doesn't exist. Sending again never helps.
• Soft bounce. The mailbox is full, or the receiving server couldn't take the message right now. A later send may get through.

The same person can soft-bounce today and receive tomorrow's email normally. A hard bounce means the address is gone.

Why it matters

• Hard bounces must stop. Mailing dead addresses again and again tells inbox providers you don't look after your list. Postmark deactivates a hard-bounced address automatically.
• Soft bounces shouldn't retire anyone. Treating a full mailbox as a dead address throws away a good subscriber.
• Repeats are a signal. An address that keeps bouncing is worth removing. Google's sender guidelines suggest unsubscribing recipients who have multiple bounced messages.

How Mimeo handles it

Your provider reports each bounce to Mimeo by webhook, with its kind. A hard bounce suppresses the person at once, so nothing else is sent to them. A soft bounce is recorded on that send and leaves the person subscribed. A late delivered event never overwrites a bounce, so a message that bounced stays bounced.

Every email and broadcast report has a Deliverability section that splits bounces into hard and soft (Reporting). People retired by a hard bounce are counted as Suppressed, apart from people who chose to unsubscribe, because "9 people left" and "9 addresses went bad" are different findings. Segments and flow conditions can ask whether someone is suppressed, which covers hard-bounced addresses. See Mimeo terms for bounces in Mimeo's own words.