This occurs when the SMTP server settings are set to use authentication but the client is not being authenticated by the email server. There may be multiple reasons for this such as:
The client (the computer hosting the TaskCentre Server installation) does not have permissions on the email server to send messages.
The username or password in the SMTP Server settings are incorrect.
Check and update your SMTP server settings:
Log in to TaskCentre.
Click Tools, Output.
Click Send Message (SMTP).
Verify that the SMTP server settings and credentials are correct. Log in to the email account listed in the SMTP settings using those credentials to confirm they work.
Also, to test the connection, click Test Connection. However, logging in to the account is a more reliable indication of whether the credentials are correct or not.
This error comes from the email server. Following verification that the SMTP server settings and credentials are correct, this should be investigated further by your IT provider.