How to fix  ‘553 sorry, unknown email address’ error in  Outlook, Thunderbird and Outlook Express

How to fix  ‘553 sorry, unknown email address’ error in Outlook, Thunderbird and Outlook Express

‘553 sorry, unknown email address’ is a common email error that happens because the recipient address is invalid. The senders would see this error in the bounce message they receive.

Error 553 is also called SMTP Error 5.5.3 and can happen irrespective of what mail server the sender is using – Exim, Qmail, Exchange, etc.

Usually this error is confined to certain recipients and isn’t a server wide one. In our Outsourced Web Hosting Support services, we encounter different variants of this error.

Today we’ll see why error 553 occurs and how to resolve it.

What causes ‘553 sorry, unknown email address’ error

The ‘unknown email address’ happens mainly due to some issue with the recipient address. We’ll see the reasons that cause this error:

1. Incorrect or invalid recipient address

Suppose the mail you sent has an invalid email address in the “To“, “CC”, or “BCC” field. You may end up getting this error message ‘553 sorry, unknown email address’.

At times, a typo in the email address of the recipient can cause this delivery failure. A missing alphabet or an incorrect character can make an email address invalid.

It is also possible that a previously-active email address gets deleted or disabled in the recipient mail server. In such cases, the recipient server would be unable to deliver that mail.


Cross-check the recipient email address you’ve given and confirm that there are no spelling mistakes or errors. If everything is fine at your end, confirm whether the email account actually exists.

2. MX entry not properly set for the domain

SMTP Error 553 can also occur when the MX entry for the domain is not set correctly. If there are conflicting entries or MX resolving to some remote server, email routing will not be proper.

Routing errors can also happen due to wrong entries in email configuration files. For instance, in Exim server, local mail delivery would be attempted for a domain listed in ‘localdomains’ file, even if its mail server is remote.

In such cases, recipient mail server would be unable to find the email account and will bounce the mail. DNS issues can also occur due to any wild card DNS or SPF record that is not set correctly.


Verify the MX records for a domain, using the command:

dig MX

If you notice DNS errors or missing MX records, then contact the recipient server support and get it fixed.

Read: 550: No such user here – How to resolve email error 550 in cPanel/WHM Linux servers

Other reasons for email error 553

Email error 553 occurs not just due to recipient email address errors. There are some sender issues that causes this error.

1. 553 error due to improper sender authentication

To safeguard from spamming and email abuse, all SMTP servers implement an authentication process. Users are allowed to connect to the mail server and send mails only after successful authentication.

This authentication process is usually integrated with the email client program such as Outlook, Thunderbird, webmail, etc. If the sender tries to send a mail without authenticating first, error message 553 occurs.

In Outlook Express, the 553 error due to improper authentication would be accompanied by the error code Error Number: 0x800CCC79.


The solution is to enable SMTP authentication in your sender’s email program and to send the mails only after validating your login details.

Here is how to configure SMTP authentication for popular email programs:

a. The steps to configure authentication in Thunderbird:

  1. In Thunderbird, under ‘Tools‘ select ‘Account Settings‘.
  2. Select ‘Outgoing Server (SMTP)‘ and click ‘Edit‘.
  3. Enable the ‘Use name and password‘ option.
  4. Enter the username and password and click OK.



Email configuration in Thunderbird


b. To enable SMTP authentication in Microsoft Office Outlook:

    1. Select the Tools -> Account Settings option.
    2. On the Email tab, click New.
    3. In the Add New Account dialog box, enter the Name, email address and password.
    4. Check the option ‘Require logon using SPA’.
    5. In the ‘More Settings’ tab, set the Outgoing server and enable the option ‘My Outgoing server (SMTP) requires authentication’. Click OK.



Microsoft Outlook – Enable authentication


c. To enable SMTP authentication in Outlook Express:

  1. In the “Tools” menu, select “Accounts…
  2. Edit the “Properties” of your email account.
  3. Click the “Servers” tab.
  4. Check the “My Server Requires Authentication” check box and the adjacent “settings” button.
  5. Make sure that the “Use same settings as my Incoming Mail Server” radio button is selected.
  6. Click “OK” and close.

ReadHow to fix Error 551 in MailEnable, Plesk, Outlook, Thunderbird and Eudora

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