The "Soft Bounce" That Isn't Soft

In email deliverability discussions, bounces are often binary: Hard (permanent failure) or Soft (temporary failure). However, this simplification hides a critical nuance known as Greylisting.

Greylisting is a method used by email servers to prevent spam. When a mail transfer agent (MTA) attempts to deliver a message, the receiving server temporarily rejects it with a "try again later" error (typically SMTP code 4xx). The theory is that legitimate servers, which conform to RFC standards, will queue the mail and retry. Spambots, built for "fire-and-forget" speed, usually will not retry.

While Greylisting is a normal part of the email ecosystem, excessive greylisting or throttling is a symptom of a deeper problem: your Sender Reputation is suspect.

Decoding SMTP 451 and 421

When you see logs filled with `451 Temporary lookup failure` or `421 Service not available`, your ISP is telling you to slow down. Why does this happen?

ISPs like Outlook and Gmail monitor the ratio of your traffic. If you send 100 emails and 10 of them hit non-existent users, the ISP's algorithms assume you are scraping data or buying low-quality lists. To protect their users, they throttle your connection.

Instead of blocking you outright (which is extreme), they simply delay your mail. This pushes your emails from "Instant Delivery" to "Delivered 4 hours later." In the context of time-sensitive password resets or flash sale notifications, a 4-hour delay is effectively a failure.

How Dirty Lists Trigger Throttling

Throttling is almost always a reaction to "Unknown User" rates.

When you attempt to email a user that doesn't exist, you force the receiving server to process a lookup request that yields no value. Do this enough times, and you look like a DDoS attack or a dictionary attack spammer.

Once you are flagged for throttling, getting off that list is difficult. You must demonstrate a sustained period of high-quality sending behavior. This means zero hard bounces.

The EmailVerifierAPI Advantage: Predictive Validation

The only way to avoid throttling caused by bad data is to ensure the data is good before the SMTP session begins.

EmailVerifierAPI helps you navigate greylisting in two ways:

1. Preventing the Trigger

By using our verification service, you remove the invalid emails (`mailboxDoesNotExist`) that cause the initial reputation drop. When you send to a 100% valid list, ISPs see you as a "High Value Sender." High value senders bypass many greylisting filters and are given priority lanes into the inbox.

2. Interpreting the Signal

Our API response includes a `sub_status` field. If we detect that a server is currently greylisting (returning `transient` errors), we flag it. This allows your application to handle these contacts intelligently—perhaps by queuing them for a later attempt or marking them for manual review—rather than blindly blasting them and damaging your IP reputation further.

Conclusion

Speed of delivery is a competitive advantage. If your emails are arriving hours late due to throttling, you are losing money. The root cause is rarely your server configuration; it is almost always your list quality. Clean the list with EmailVerifierAPI, and you remove the friction between your server and the inbox.