Bounce rate

The published bulk-sender guidance from the major providers puts the acceptable bounce rate low — a sustained rate above roughly five percent is the point at which a sender starts being treated as a list problem rather than as an individual with a few bad addresses. Above that, filtering escalates quickly, and domain-level blocks follow.

It is worth being precise that this is not an opinion held by any vendor. It is the number enforced by the receiving side, and it is therefore the number to design around.

A hard bounce should suppress the address immediately and permanently. Sending again to an address that does not exist has no upside and a direct reputation cost, and it is the single most common avoidable mistake in outbound.

A soft bounce should be retried a small number of times and then treated as hard. The failure mode is retrying indefinitely — a mailbox that has been full for three months is not going to accept the fourth attempt either.

Two bounces out of five sends is a forty percent rate and means nothing at all. A rate computed on a small sample will trigger any threshold you set, which is why a sensible system requires both a rate over the limit and enough absolute events for the rate to be real before it acts.

Measure against messages that actually left, too. Queued and failed-to-send messages say something about your infrastructure, not about the recipient's opinion of you.

SalesShift pauses a mailbox when its bounce rate reaches 5% of sends and at least five bounces have occurred — both conditions, so a small sample cannot trigger it. Bounce rate is one of four inputs to the per-mailbox health score, and hard bounces are added to the workspace suppression list.