Error: NDR - Authentication Credential Invalid (Exclaimer Cloud – Signatures for G-Suite)


You are using Exclaimer Cloud – Signatures for G-Suite, and start to receive bounce-backs from sent messages, stating:

535 5.7.8 Authentication credentials invalid [Response from was 534 5.7.14 q6sm95039edh.25 - gsmtp] (e2b5e025-d561-4261-b5ac-e47a87b7179d,92611451-15de-4cbf-ba58-879f67066cc6)


This issue is caused by the SMTP authentication failing, when your messages attempt to proxy through our service back to G-Suite.


The most common cause of this issue, is that the user account used for the SMTP Authentication has an expired password, or the setting for the authentication requires MFA (which is supported via the use of an App Password).

To confirm either of the above, please follow the steps below:

Reaffirm the password used for SMTP Authentication

  1. Login to Exclaimer Cloud here.
  2. Launch your subscription:
  3. Click the ‘Hamburger’ icon in the top right and go to Settings:

  4. Select the Mail Flow tab, then click Edit:


  5. Enter credentials of the SMTP Authentication account (we recommend creating a dedicated account for this process):


  6. Click Next. A final page is displayed, directing you to complete G Suite configuration:


  7. Leave this page open (as you may wish to copy/paste these settings), and follow instructions detailed in Exclaimer Cloud - Signatures for G Suite: Server Side Setup (Admin Console).

  8. When you've finished this configuration, return to this wizard and click Close.

Set less secure apps on a per user basis  

  1. Log into the Google Admin Portal here.

  2. Navigate to Security > Basic Settings > Less Secure Apps.

  3. Click on Go to settings for less secure apps >>:

  4. Change Disable access to less secure apps for all users to Allow users to manage their access to less secure apps:

  5. Click Save:

Ensure your account has access

  1. To enabled less secure apps for the user, go here.

  2. Set Allow less secure apps: to ON:

Haven't found a solution to your problem?

Please click the button below, login to the Exclaimer Cloud portal and raise a support ticket. A member of the Exclaimer Support team will then be in touch.

Raise a support ticket