What causes soft bounces?
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Soft bounces are temporary failures. The address exists and the server is reachable, but something stopped the message from being delivered right now. Your ESP will retry for a day or two, and often the message gets through on a later attempt.
The most common causes: the recipient's inbox is full (SMTP 452 or 4.2.2), the receiving server is temporarily overloaded or offline, the server is rate-limiting your IP because you're sending faster than it wants to accept, or the receiving domain is using throttling as a policy to control inbound volume.
One cause that surprises senders: if your authentication isn't clean, some servers respond with a temporary failure code instead of an outright rejection. Check that your SMTP error codes are actually temporary (4xx) and not permanent (5xx) being misclassified by your ESP as soft bounces.
An isolated soft bounce is usually nothing to worry about. The problem is patterns. If the same address soft bounces repeatedly over multiple campaigns, that's a sign the mailbox may be abandoned, over quota permanently, or blocking you. Most ESPs convert repeated soft bouncers to hard bounce status after a certain threshold. If yours doesn't, set up your own rule and move those addresses to a suppression list. Consistently mailing addresses that keep soft bouncing hurts your overall bounce rate and your reputation.
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