Documentation
Temporary delivery failure: 4xx.
The receiving server could not take the message right now, and said to try later.
What it means
A code beginning with 4 is a temporary failure: the receiver is busy, rate-limiting you, greylisting a first-time sender, or briefly unavailable. Mailtruss's delivery network retries for a while before it reports a temporary failure as a bounce, so by the time you see one, the first retries have already happened. It does not retry again by itself.
Typical codes and wording
4xx, 4.x.x (for example 451 4.7.1, 421, 450)
Common causes
- The receiving server was overloaded or down for maintenance.
- The receiver is throttling mail from your sending address or network.
- Greylisting: some receivers refuse an unknown sender once and accept the retry.
- A temporary DNS problem on the recipient's domain.
What to do
- You can send again later. A few hours is usually enough.
- If it keeps happening for one recipient domain only, the problem is likely on the receiver's side.
- If it happens for many domains at once, check your recent sending volume and complaint rate, which can make receivers throttle you.
- Use an idempotency key on your own requests so that sending again after a timeout can never deliver the message twice.
What Mailtruss shows
Titled "Temporary delivery failure", marked safe to retry. A temporary failure does not add the address to your suppression list.
Is it safe to retry?
Yes, later.