This is called a “bounce message” that indicates your email could not get to the recipient. Sometimes this is caused by a temporary problem with mail servers on the internet, but usually it is because of a problem with the email itself. Typically, the bounce message will include an error indicating why the original email could not be sent.
Here are some common errors you may encounter:
- “Unknown mailbox” means that the email address does not exist on that domain. Most likely, the address is spelled incorrectly.
- “Account over quota” means that the email address you are sending to does not have enough space in the mailbox to receive your email. You will need to wait until the recipient has cleared enough space in their mailbox for your message.