The -ERR NTLM Failure error is usually caused by a mail server configuration issue.

Some mail servers advertise NTLM as a supported authentication method even though NTLM authentication has been disabled. When eM Client attempts to use it, the server rejects the connection and returns this error.

The proper fix is to update the mail server configuration so it no longer advertises NTLM if it is not supported. Please contact your mail server admin or email provider to have this corrected.

If the server cannot be updated, our support team can provide a workaround.
Before contacting us, please enable logging for the affected account under Accounts > Diagnostics, save the changes, restart eM Client, reproduce the issue, and then send us the logs. You can find the log file location under Settings > Advanced. The workaround needs a specific alternate authentication method to be used, so we cannot provide advice without the log.