SMTP Auth is just the protocol extension that allows a client to use Authentication when sending a message . The shared message means the authentication failed for the specified user "email@example.com".
Is this a legitimate user in your network? If not, it is not an issue they tried to login with some random password and it failed. If it is legitimate user , please validate their credentials.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.