# SMTP Errors, Deferrals, and Throttling > SMTP errors and email delivery service responses explain 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 free tool, pair the tool result with the relevant issue or step section from this guide. ### SMTP Errors, Deferrals, and Throttling SMTP errors and email delivery service responses explain whether a delivery problem is temporary, permanent, or caused by rate limits. When an email cannot be delivered normally, your email delivery service or receiving server may return an SMTP response. These responses are clues. They often explain whether the issue 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 saying, in effect, try again later. This is often called a deferral or temporary failure. Examples include mailbox provider throttling, temporary server problems, greylisting, or 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 mail down. It 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. Retries should be controlled by your email delivery service. Manually resending a large campaign because the first send was deferred can make the problem worse. First read the error, find the cause, and let service retry logic work where appropriate. Not every error appears inside Mailrith with full detail. Sometimes you need the dashboard in your email delivery service because that service sees the SMTP conversation and queue state. 1. Find the exact error in Mailrith, the dashboard in your email delivery service, or the bounce message. 2. Look at the first digit of the SMTP code if one is available: 4xx is usually temporary, 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. 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 recent campaign behavior. 7. If many addresses hard bounce, suppress those subscribers and inspect the list source. 8. Resume sending with a smaller engaged subscriber group after the root cause is fixed. - 4xx usually means temporary. Wait, investigate, and let email delivery service retries happen. - 5xx usually means permanent. Treat it 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 complaints, bounces, volume, and list source. - Email delivery service dashboards often have more detail than Mailrith can show. - Do not retry large sends blindly after deferrals or blocks. > The error message is part of the diagnosis. Preserve it before changing settings, deleting a connection, or retrying 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 sending, look at delivery, opens, clicks, unsubscribes, bounces, and complaints together. - [Handle Bounces, Complaints, and Unsubscribes](https://mailrith.com/guides/handle-bounces-and-complaints.md): Delivery events protect future sends by keeping risky addresses out of normal campaigns. - [Troubleshooting Common Sending Problems](https://mailrith.com/guides/troubleshooting.md): Most sending issues can be narrowed down by checking setup, subscribers, content, email delivery service, and subscriber status.