Some users do not get email notifications as expected, and some do, but no errors are found in the server log.
In the client log from which the email should be originating, the following message is found:
EXCEPTION OCCURRED:SmtpException The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.1 Client was not authenticated
Email Infrastructure team in your organization must be contacted to resolve this problem. SMTP configuration needs to be revised to open up security for users requiring access to send via SMTP.
In this case, client had multiple Exchange servers, 3 on Exchange 2010 work fine and 2 on Exchange 2013 were causing the problem.
Receive Connector feature on Exchange 2013 servers were altered to include Exchange users in the Permission groups on the default FrontendTransport Receive Connector.
https://communities.bentley.com/products/assetwise/asset_performance_management/w/wiki/27144.email-notifications-not-being-sent
https://communities.bentley.com/products/assetwise/asset_performance_management/w/wiki/27900.utility-to-test-smtp-send