Why is there a delay in receiving bounced messages?

Customers occasionally ask why they received a undeliverable “bounce” message for an email they sent days before.

Most of the time, you’ll see any undeliverable “bounce” as soon as you send a message. But sometimes, the receiving email server indicates it’s having a temporary problem — that it’s unable to accept mail now, but that it wants our servers to keep trying, because it should be possible to deliver it in the future.

For example, this happens if the receiving mail server is undergoing scheduled maintenance, or experiencing temporary downtime or another technical issue that should be resolved soon. This is common — and in most cases, the message is successfully delivered with a short delay. (It wouldn’t make sense to “bounce” a message as undeliverable if the receiving mail server is just offline for a few minutes.)

However, there are some cases where the receiving mail server has an extended period of downtime, or always indicates that it’s having a “temporary” failure. When this happens, our servers will keep trying for the Internet standard of up to 5 days before finally sending a “bounce” notice that the message was undeliverable even after retrying.

All of this is controlled by the receiving server and the response it sends, so there isn’t anything that can be done to speed up or change this process for individual messages. But it’s rare for a real address to have failing servers for 5 straight days: if a real domain name has technical problems, someone will usually notice it and fix it before that.

Because of that, these kinds of bounces usually happen only when messages are sent to invalid servers that nobody is monitoring, particularly ones with “typos” in the domain name. For example, if you send a message to an address @gmai.com instead of @gmail.com (note the missing letter in “gmail”), it might take a while to bounce. The “typo” domain name exists on the Internet, but because it doesn’t usually receive mail, nobody notices or fixes the “temporary” problem its mail server has.

Five-day-old bounces are a common problem if your email address was compromised and used to send spam that ends up at non-working email servers. If that happens, it usually doesn’t mean there’s a new problem — it’s just the end of the original problem.