SMTP and IMAP Authentication Errors

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What is authentication?
When you use email in, IMAP or SMTP is used to send or receive the email. Communication is opened between the servers and your mail server, and Usernames and Passwords are used to authenticate the user. All IMAP servers require authentication, but some SMTP mail servers do not require authentication. Where required, the authentication is in the form of a valid username and password. The authentication is the handshake between the servers and your mail server, to verify that we are who we say we are and have access to send/receive from the mailbox.

For Inbound Mailboxes (IMAP), this authentication is set up in Admin→Channels→Email→Mailboxes, in each individual mailbox.

For Outbound Mailboxes (SMTP), this authentication is set up in Admin→Settings→Outbound Mailboxes, in each individual mailbox.

The type of authentication that is used for your Outbound Mailbox is determined by your email administrator, if you are unsure we recommend giving your email administrator access to this admin screen to see the available options, or let them know what is available (NONE, PLAIN, LOGIN, CRAM_MD5).

What are the symptoms of authentication errors?
  •     Emails being bounced back or unable to be sent.
  •     Emails being stuck in the Outbox.
  •     Customers not receiving their emails
  •     Case not being created.
  •     Emails not being pulled in.
  •     Similar error message to the ones below appearing in your error logs:
         →LOGIN Failed.     
         →535 5.7.3 Authentication unsuccessful
         →551 5.1.1 No such user found     
         →504 5.7.6 Unrecognized authentication type     
         →535 5.7.8 Error: authentication failed     
         →535 5.7.8 Error: authentication failed: authentication failure\r\n     
         →550 5.7.1 Client does not have permissions to send as this sender\r\n
         →535-5.7.1 Please log in with your web browser and then try again.

You should see one of these errors, or something similar in your system logs. If you see errors but cannot make out the error, you should reach out to support for guidance as we may be able to do more in depth analysis of your email issue. Keep in mind that these are generic error messages that are happening at the SMTP or IMAP level and are not specific to any one email server type or brand.

How do I correct the authentication error?
This is a mismatch between what you have in vs what you have on the mail server regarding authentication for this mailbox. More often than not, someone changed the password to the mailbox on the mail server and didn't tell you. The easiest way to test this is to try and log into the mailbox on the mail server (using a webmail client, Outlook, Thunderbird, or other client application)  using the EXACT mail server name, username and password. If you cannot log in with your mail client, then you won't be able to log in with Once you have the correct login credentials, then you should enter those in the appropriate Inbound and Outbound mailbox settings within Desk.

In some cases, the internal mail server name that you use is different than the external mail server name that you use. You can check what the outside world (including see's as the MX Record for your domain a using their Supertool. Type in your domain name ie.: (not the full mail server name) to see what the outside world see's as your mail server name. This is the server name that you should be using in


If you are using a Google Apps or Gmail account, you may need to unlock the Captcha on their site using these instructions from the support site

If you can log in with your webmail or you mail client using the exact settings that you have in, but are still receiving an authentication error, then it is highly recommended that you get your email server administrator review the Outbound Mailbox settings and locate the failure point of the authentication by reviewing the mail and network logs, and possibly the security logs on the mail server.