Understanding 4xx Deferrals

Let us take a moment to talk about SMTP status codes. Specifically, the kind that start with the number 4.

Say you've just sent out your latest email, whether it be a newsletter or marketing campaign. You know that it should land in the inbox right about ... now. But this time ... nothing. It's not in your Gmail, Yahoo, or Outlook test accounts. No bounce. No delivery. Just ... nothing.

So where is the mail?

If you don't have visibility into your email sending platform's MTA (Mail Transfer Agent) queues, it's hard to say. Some email platforms show queue and delivery info. Many don't.

If you do have access, you might see a big backlog of email messages queued up, waiting to get into a major mailbox provider. And each message will likely show a recent SMTP status response from that provider.

Sometimes, that status will look like this:

• Yahoo: 451 Resources temporarily unavailable. Please try again later
• Microsoft: 451 4.7.500 Server busy. Please try again later
• Gmail: 421 4.7.0 Temporary System Problem. Try again later

This doesn't mean they hate you

These aren't reputation problems. They're system problems. And there are three clear signs telling you the issue is on their (the mailbox provider's) end:

1. The numeric code starts with 4. SMTP status codes starting with 4 mean "temporary failure," technically these give a "transient negative completion reply," if you want to quote the RFC. (Codes that start with 5 are permanent failures — those are bounces.) A 4xx inherently means: try again.
2. The text says "resources temporarily unavailable," "server busy," or "system problem." That's them saying "we've got an issue." There's no reference to spam or reputation there.
3. They tell you to try again later. It might sound like the email version of "your call is very important to us," but they do mean what they say here. The mailbox provider does, literally and actually, want you to try (retry) delivery of that email message again later.

Put all that together and you've got your classic 4xx deferral.

Side note: Sometimes, a provider might use a 4xx code to slow-walk you for reputation reasons. For example, a 421 with wording like "rate limited due to user complaints" might still be a reputation issue in disguise. So don't assume every 4xx is innocent, but most are.

What happens next?

Your email platform will keep retrying delivery. Those messages sit in the queue and wait. One of two things will happen:

• The messages are eventually delivered on a retry (a future delivery attempt).
• The platform gives up after too many retries or too much time has passed.

How long they'll keep trying depends on your sending platform. Amazon SES will retry for up to 14 hours, for example. Many others will retry for up to 72 hours. A few might go 4-5 days. Once the receiving provider fixes the issue, mail will flow again. The queues will drain. Mail will start landing in inboxes like usual.

How long in-between each delivery attempt (retry) is also up to the sending platform. The RFC says that "in general, the retry interval SHOULD be at least 30 minutes," but suggests that "more sophisticated and variable strategies" may be useful, and indeed, various platforms have different intervals. 

Does this hurt my sending reputation?

Nope. System deferrals like these aren't aimed at you, and they don't damage your sender reputation.

Could delayed delivery affect open rates? Maybe. If your readers tend to check their email early in the morning, and your mail shows up in the afternoon instead, you might see a dip. But a temporary drop in engagement like that isn't going to blow up your sending reputation.

Also, keep in mind: If Gmail is having problems, it won't affect delivery to Yahoo or Outlook. These issues are narrowly provider-specific.

Is this my ESP's fault?

Almost never. Unless your ESP, CRM or newsletter platform has a terrible reputation across the board, this isn't on them. It's a provider-side system issue. Nothing more. Keep an eye out for references to shared IP address reputation or shared domain reputation, as those are indicators that an ESP might need to better police their clients to keep shared resources clean and keep shared reputations positive. But this is generally rare.

TL;DR and let it be

4xx SMTP deferrals happen. They're temporary. They usually don't mean that you're a spammer. They don't hurt your reputation. They just mean the receiving system is having a bad day. Everybody's favorite email protocol (SMTP) was built with this kind of thing in mind. Your ESP will retry. And most of the time, the email eventually will flow.

Either way, you're not doing anything wrong.

Keep calm and carry on.