How to track mailbox-full or temporary deferrals?

Mailbox full messages return specific soft bounce codes such as 452 4.2.2 Mailbox full. Temporary deferrals return codes such as 421 4.7.0 Try again later. Track these codes over time and identify patterns. If many soft bounces cluster around certain domains, throttling or rate limits may be involved.

Your ESP usually records these events in bounce logs.

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