What does Delivery Status Notification '550 5.1.1 User unknown (in reply to RCPT TO command)' mean?

Problem:

You are trying to send an E-Mail but you receive a Delivery Status notification response like this:

This is the mail system at host techoverflow.net.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<[email protected]>: host mail.recpient.com[213.216.0.1] said: 550 5.1.1 User
    unknown (in reply to RCPT TO command)

Solution

If the response contains a line like this:

<[email protected]>: host mail.recpient.com[213.216.0.1] said: 550 5.1.1 User unknown (in reply to RCPT TO command)

it means that the recipient’s email address (check the start of the line - in this case it’s [email protected]) does not exist. Most likely

You used a wrong or mis-spelled email adress (check the address for typos) or
the person you sent the mail to doesn’t exist any more at the company you’re trying to send the mail to
the domain does not match - e.g. company.com instead of company.net - or copmany.net instead of company.net

Note that if you sent the email to multiple recipients (e.g. using CC), the other recipients will have received the mail if you don’t get a separate status notification or they are listed in that Delivery status notification as well. Note that one Delivery status notification E-Mail might have multiple 550 5.1.1 User unknown lines, so be sure to check the Delivery status notification carefully.

Note that in some cases it will take a couple of minutes until the Delivery status notification arrives in your Inbox, or it might end up in your Spam folder, so be sure to check there as well.

Tip: In case you don’t have another E-Mail address at the company you’re trying to send E-Mails to, try these addresses:

or check the company’s Imprint or Privacy page, which often contains an active E-Mail address.