Soft bounces usually carry an SMTP code starting with 4. The sending server keeps the message and retries it over several hours or days before giving up. Typical causes are a full mailbox, a server that is down or busy, a message that is too large, or a receiver slowing you down because you are sending a lot to it at once.
452 4.2.2 The email account that you tried to reach is over quota.
421 4.7.0 Try again later, closing connection.
When a soft bounce becomes a problem
- The same address soft bounces on every send for weeks. Treat it as a hard bounce and stop.
- Many addresses at one provider defer at once. The provider is throttling you; slow down and check your reputation there.
- Your mail is deferred with words like policy, reputation or spam in the text. That is a deliverability signal, not a full mailbox.
Soft bounces in cold email
A few soft bounces are normal and need no action. A rise in deferrals from one provider, while others are fine, is often the first sign that your mailbox is sending too much or that recipients there are marking you as spam. Spread volume across more mailboxes, keep each one near 20 cold emails a day, and keep email warmup running.
ZicReach reads placement by mailbox provider from engagement, so a provider where people stop opening and replying stands out next to its usual level, with a plain next step. Combine that with routing rules by provider in AI Email Marketing, and check the full picture on the warmup and deliverability page. For the permanent kind, see hard bounce.