Classify the rejection
Separate invalid-recipient responses from policy, authentication, content, and size rejections. The response may come from the recipient server or an intermediate gateway, so preserve the full diagnostic and timestamp when investigating.
Check the relevant control: recipient spelling and mailbox status for address errors, sender authentication and domain alignment for policy responses, and message format or attachment constraints for content or size issues.
Correct and retry selectively
Fix the underlying cause before sending again. Retry transient service or rate-limit responses with an appropriate delay, but suppress a confirmed invalid address unless the recipient provides a corrected one.
Track bounce categories and rates over time without retaining unnecessary message content. If many recipients at one domain reject mail, investigate the shared sender configuration or reputation signal rather than repeatedly resending individual messages.