Explanation of the email sending process / SMTP

What happens to mail sent to your domain when a mail server is briefly unreachable? And to mail you send when the recipient's server is down? In short: mail doesn't just disappear. Here's how it works.

Mail delivery is retried

Mail travels between mail servers using the SMTP protocol. If delivery fails because the receiving server is briefly unreachable or temporarily refuses the mail, the sending server keeps the mail in a queue and tries again later. Frequently at first, then with increasingly longer pauses.

• How often and for how long varies per mail server. Usually it's a few days.
• Our own servers retry every fifteen minutes for the first two hours, then less and less often, up to four days after sending.
• If delivery still fails within that time, the sender receives an error message (bounce) stating the reason. Some servers also send an interim message saying the mail has been delayed.

So if a mail server has been offline for a while, the mail will still arrive once it's working again. And if delivery really isn't possible, the sender will be notified.

Mail you send yourself

The same applies to mail you send through our servers. If the mail can't be delivered, you'll get an error message back. You can find out how to read it in Mail delivery failed: how to read the error message and what the codes mean. If you get a message saying the mail has been delayed, don't send it again: the server will keep trying on its own.

An old email that's only arriving now?

If an email arrives late because a server was unreachable, you won't always notice right away. Most email programs show the date the mail reached you, not when it was sent. The headers of the message show the actual time it was sent and the route the mail took. Read Viewing and reading the headers of an email.

Want to learn more? Take the free course Getting your email in order, lesson 1.1 The journey of an email and lesson 1.2 SMTP: the conversation between mail servers.