Outbound Glossary
Soft bounce
A temporary rejection, like a full mailbox or a throttling server. Harmless once, a warning sign when it clusters at one provider.
A soft bounce means the message could not be delivered now but might be later, so most tools retry for a while before giving up. Causes range from a full inbox to a server deciding you are sending too fast.
The number alone tells you little. What matters is the pattern: soft bounces spread evenly across providers are noise, while soft bounces concentrated at one provider usually mean that provider has started throttling you, and that is the moment to slow down rather than push through.