Greylisted: '450 Try Again Later' and What It Means for Cold Email

A "450 try again later" reply, often with the word greylisted, is a temporary refusal that the receiving server gives to a sender it does not know yet. It is not a verdict on the address, and you should not remove the address. A proper sending tool retries on its own, and the message normally goes through on a later attempt. The real risks are tools that do not retry, and retries that never get past the filter, which end as expired messages. If you want to know in advance which addresses answer with a definite yes and which stay vague, verification shows it: SimpleVerifier Pro is $39 a month, the first 100,000 checks a month at full speed, then slower, never cut off, with 100 free checks.

What greylisting is

RFC 6647, the document that describes the practice, defines greylisting as temporarily degrading service for unknown email clients as an anti-abuse measure. In the narrow sense it means a server returns a 4xx temporary failure to an unfamiliar source. The reasoning in the RFC is that spamware typically does not retry after a temporary failure, while legitimate mail servers do. The RFC also notes that some clients do not retry messages at all, and that a client needs to be equipped to retry to be considered fully capable. We read it on 10 November 2026.

How it works in practice:

1. Your server connects and tries to deliver. The receiving server has not seen this combination of sender and recipient before.
2. It replies with a 4xx temporary failure and closes the attempt.
3. Your server queues the message and tries again later.
4. If the retry arrives within the server's window, the server accepts it, and usually remembers the source so later mail passes straight through.

On the reply code, RFC 6647 says that under SMTP the only two reasonable choices are 421, if the server wants to end the connection immediately, and 450 otherwise. The text it returns is optional, and some implementers prefer to say nothing about greylisting so spammers get no hint about retry timing. So you may see a plain "450 try again later" with no mention of greylisting, or something clearer.

The windows are configurable. RFC 6647 suggests the default window should be from one minute to 24 hours, with a retry outside it treated as a new message. That means your first message can be held for minutes or hours, which is fine for a newsletter and awkward for a time-sensitive email.

What it looks like in your sending tool

Different tools show it differently:

• Delayed or deferred: the message shows as queued, delivered late, or retrying. This is the normal case.
• Soft bounce: some tools count the 4xx as a soft bounce. See soft bounce handling.
• Failed after retries: if the retries never pass, the final report says the message expired. RFC 3463 defines X.4.7 as delivery time expired, where the message was considered too old by the rejecting system. Exchange Online reports it as "Message expired".
• Nothing visible: some tools hide the retry and only show delivered or not delivered.

SMTP deferral vs bounce goes through how to read each status, and the SMTP transcript explainer reads a pasted transcript line by line.

Greylist, rate limit or reputation block?

A 450 or 421 can have more than one cause, so confirm before assuming. Here is how to separate them:

Sign	Likely cause
First email to a new contact is delayed, later ones pass	Greylisting
Many addresses at one domain deferred at once, text mentions rate or volume	Rate limiting
Text mentions reputation, IP or low reputation	A reputation filter, not greylisting
Defers every time, never delivers	Block or a server that does not accept you

Our posts on Gmail's 421 4.7.28 unusual rate and 450 4.2.1 receiving too quickly cover provider-specific cases of throttling that look similar.

Step by step

1. Do nothing first. Give the retry a chance. Most greylisted mail delivers without you doing anything.
2. Check your tool retries. Find its retry setting. If it marks a first-attempt 4xx as final, change it, or the greylisted mail is lost.
3. Check the final status after a day. Delivered late is fine. Expired means it never cleared.
4. Do not resend manually in a flurry. A new message may be treated as a new unknown message and restart the delay.
5. If the same domain never clears, treat it as a blocking or reputation issue, and look at the text for clues. Do not push more volume at it.
6. Mark the address risky, not invalid, if your list tool allows it, and test with a small batch next time.
7. Check how many are affected. If a large share of one send was deferred, slow the send and look at the sending reputation. See what to do after a high bounce rate.

What greylisting means for cold email timing

Cold email often relies on a first message arriving at a time that suits the recipient. A greylisted first message can arrive later than planned, and the recipient's first impression of you is a message that came at an odd hour. That is minor. The real cost is for a sequence: if step one is delayed by many hours, step two may arrive close behind it. Check your sequence spacing, and expect some first sends to land later than scheduled.

Where verification fits

A mailbox check is a short SMTP conversation with the recipient's server, so it can run into the same kind of temporary refusal. When a server will not give a definite answer, SimpleVerifier labels the address risky instead of guessing. Risky is not bad: it means "send, but carefully". That is the honest answer, because an address behind a greylist may be perfectly fine.

So verification helps on two sides:

• It removes the addresses that are definitely dead, so the only deferrals left in your send are the uncertain ones.
• It tells you which addresses to put in a small first batch, so a delayed delivery does not affect the rest of the send.

Our guide to accept-all vs unknown results explains how to treat risky rows, and verified emails that still bounce explains why no check gives a guarantee.

Try one address with the free email checker, or run a list through /verify.

How SimpleVerifier fits

SimpleVerifier Pro is $39 a month. The first 100,000 checks a month run at full speed, then slower, and you are never cut off. You can try 100 free checks first, no card. See pricing.

What not to do

• Do not delete greylisted addresses. They are temporary, not dead.
• Do not turn off retries to "avoid bounces". That loses the mail.
• Do not whitelist-chase. You cannot ask every recipient's server to trust you. Retry, warm up and keep your reputation clean.
• Do not resend instantly. It can restart the delay.

Bottom line

Greylisting is a temporary 4xx that tests whether your server retries. Let the retry happen, confirm your tool retries, and look at the final status after a day. Treat addresses that never clear as risky rather than invalid, and use verification to remove the clearly dead ones first. The bounce doctor helps when you cannot tell which type of refusal you are seeing.