# SMTP Errors, Deferrals, and Throttling > SMTP errors and email delivery service responses show whether a delivery problem is temporary, permanent, or caused by rate limits. - Human page: https://mailrith.com/guides/smtp-errors-deferrals-and-throttling - Markdown page: https://mailrith.com/guides/smtp-errors-deferrals-and-throttling.md - Category: Learn and Fix - Reading time: 4 min read - Related keywords: SMTP Errors Deferrals and Throttling, SMTP Errors Deferrals and Throttling guide, Learn and Fix, Learn and Fix guide, email sending guide, email marketing guide, email deliverability guide, Bounces, Sending Volume and Warmup, Troubleshooting Common Sending Problems ## AI Agent Notes - Use this page as plain-language guidance for the specific email sending issue named in the title. - Preserve the distinction between Mailrith, an email delivery service, DNS, and inbox providers when explaining fixes. - When a user is running a Mailrith free tool, pair that specific free-tool result with the relevant issue or step section from this guide. ### SMTP Errors, Deferrals, and Throttling SMTP errors and email delivery service responses show whether a delivery problem is temporary, permanent, or caused by rate limits. When an email cannot be delivered normally, your email delivery service or the receiving server may return an SMTP response. Use the response as a clue. It often explains whether the delivery problem is temporary, permanent, reputation-related, authentication-related, or caused by sending too fast. A 4xx response usually means a temporary problem. The receiving server is telling the sender to try again later. This is often called a deferral or temporary failure. Examples include mailbox provider throttling, temporary server problems, greylisting, and rate limits. A 5xx response usually means a permanent failure. The address may not exist, the domain may reject the message, authentication may fail, or the receiver may block the sender. These failures often become bounces. Throttling means the receiver or your email delivery service is slowing down email delivery. Throttling can happen when volume rises quickly, complaint rates are high, authentication is weak, the sender is new, or the service is protecting its own systems. Your email delivery service should control retries. Do not manually resend a large campaign only because the first send was deferred; doing so can make the problem worse. First read the error message, identify the cause, and let the service retry when retry logic applies. Mailrith may not show every error with full detail. Sometimes you need to check the dashboard in your email delivery service because that service sees the SMTP conversation and queue state. 1. Find the exact error message in Mailrith, in the dashboard in your email delivery service, or in the bounce message. 2. If an SMTP code is available, check the first digit: 4xx is usually temporary, and 5xx is usually permanent. 3. If the message is deferred, do not immediately resend the whole campaign. Check whether your email delivery service is retrying the send. 4. If the error mentions authentication, inspect SPF, DKIM, DMARC, and alignment. 5. If the error mentions rate, quota, policy, or reputation, review volume, complaints, bounces, and email delivery service limits. 6. If many addresses fail at one mailbox provider, check that mailbox provider's sender guidance and review recent campaign behavior. 7. If many addresses hard bounce, suppress those subscribers and inspect the list source. 8. After you fix the root cause, resume sending with a smaller group of engaged subscribers. - 4xx usually means temporary. Wait, investigate, and let email delivery service retries run. - 5xx usually means permanent. Treat the response as a bounce or policy rejection unless the email delivery service says otherwise. - Throttling often means the receiver wants slower or healthier traffic. - Authentication errors should lead you to SPF, DKIM, DMARC, and alignment checks. - Policy or reputation errors should lead you to review complaints, bounces, volume, and list source. - Email delivery service dashboards often show more detail than Mailrith can show. - Do not retry large sends blindly after deferrals or blocks. > The error message is part of the diagnosis. Save it before you change settings, delete a connection, or retry the campaign. Related resources: - [Bounces](https://mailrith.com/guides/bounces.md): Understand hard and soft delivery failures. - [Sending Volume and Warmup](https://mailrith.com/guides/start-small.md): Avoid sudden volume patterns that can cause throttling. - [Troubleshooting Common Sending Problems](https://mailrith.com/guides/troubleshooting.md): Walk backward through setup, email delivery service, subscribers, and inbox symptoms. ## Related Guides - [Read the Results](https://mailrith.com/guides/read-results.md): After a campaign sends, review delivery, opens, clicks, unsubscribes, bounces, and complaints together. - [Handle Bounces, Complaints, and Unsubscribes](https://mailrith.com/guides/handle-bounces-and-complaints.md): Delivery events help protect future sends by keeping risky addresses out of normal campaigns. - [Troubleshooting Common Sending Problems](https://mailrith.com/guides/troubleshooting.md): You can narrow down most sending issues by checking setup, subscribers, content, email delivery service, and subscriber status.