Sent email in comes back "delivery failed"

Delivery failures, or emails that bounce or come back as undeliverable, can happen for several reasons. When you send an email and receive a delivery failure message, there could be several reasons. We've listed the most common error messages below, along with suggested resolutions.

Temporary errors

Temporary error messages can include the following.

  • Can't connect to remote server/message expired

  • No adequate servers

  • Connection timed out

  • Resources temporarily unavailable

  • Out of memory

Try this fix:    These types of errors should fix themselves over time. If your error message says that the recipient's email provider will keep trying to deliver your message, you don't need to take any further action. Otherwise, wait a short time and then resend the message.


If you receive an error that a message is undeliverable, this could be due to the sending server trying to deliver the message but the action didn't complete before the message expired. This error typically indicates a problem on the receiving server.

Try these fixes:    Make sure the recipient address is valid. Reduce the number of recipients in the message. If you received this error when sending a message using Outlook or another email app, try using to send the message instead.

Mailbox not found, Invalid mailbox, or User unknown

If you receive one of these messages, the address you used doesn't exist. This could be because the recipient's address has changed or the recipient's email account has moved.

Try this fix:    Check the recipient's email address for typos. If you have another way to get in touch with the recipient, see if they have a new email address.

Mailbox unavailable

Often, this error message has the same cause as the Mailbox not found message. However, occasionally there's a problem with the recipient's email account that will resolve itself over time.

Try this fix:    Wait a short time and try to send your message again. If that fails, check the recipient's email address for typos. If you have another way to get in touch with the recipient, see if they have a new email address.

Mailbox full or Quota exceeded

This message means that the recipient has too many messages in their inbox. Many email systems prevent users from receiving any new mail until they reduce their mailbox size.

Try this fix:    If you have another way to contact the recipient, let them know that their mailbox is full.

Host unknown or Domain lookup failed

If you receive one of these messages, the domain for the recipient's email address doesn't exist. If their email address is, the domain is

Try this fix:    Check the recipient's email address for typos. Ensure you have the current address and that their email service provider hasn't made a change to their domain name.

Message too large

Most email service providers impose a limit on the size of messages their users can send or receive. Your message is too large for the recipient's email server.

Try this fix:    Instead of sending large attachments via email, upload them to OneDrive and send the recipient a link.

Errors that include "blocked" or "listed in" and references to sites like "spamcop," "dynablock," "blackhole," or "spamhaus"

These terms are used when your message has been intentionally blocked by a spam or blacklist filter on the recipient's email server. The recipients server believes that the mail server or mail account that you're using is a source of spam.

Try this fix:    You can contact the support team for the recipient's email provider and request to have your account or domain unblocked.

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