You've probably been there: you've tried so hard to write a good email, then you get a reply saying it hasn't been sent to the recipient. Or you receive a notification that an order confirmation hasn't reached your customer. The email will then say "Undelivered Mail Returned to Sender" or "Delivery Status Notification (failed)", for example. 

Of course, these are not the nicest messages, but I wouldn't just move them to the trash either. After all, you will always find a reason for the error message in these emails. It's therefore a good idea to read these emails carefully and reply if necessary. 

Some common error messages:

User Unknown

This message exists in different variants. For example, it can also occur if it mentions an e-mail address. In any case, it indicates that the mailbox does not exist. This could be an indication of a typing error.

Host or domain name not found. Name server error for name =...type host...: host

This means that the domain and the mailbox no longer exist.

Message size exceeds fixed limit

This notification indicates that the message you are trying to send is too large. Check your attachments and the email and try to reduce the size of the message if possible. A safe size is 10 MB.

Exceeds quote / Quota exceeded (Mailbox for user is full)

This means that the recipient's mailbox is (temporarily) too full. You can try again at a later time. If you know the person, you can always let them know that their mailbox is full and they can no longer receive mail.

Message rejected due to content restrictions

This notification means that the message has been rejected due to its content. This is probably due to a spam filter.

Message blocked due to virus content of the message

This means that the message has been rejected due to its content. This is probably due to a virus filter. You may receive an automatic reply describing what to do.

Connection timed out

In this case, the receiving mail server does not respond to our mail server. This is of course very annoying and may be due to an incorrect configuration or firewall setting on the receiving server.