Microsoft is shutting down legacy authentication. You might want to check if the legacy authentication is already shut down in the clients tenant.
Log in as adminhttps://admin.microsoft.com > Settings > Org settings > Modern authentication. Make sure basic authentication protocols are selected.
Also check settings per user. Go to https://admin.microsoft.com > Users > select user > click on Mail > Manage email apps and make sure you have selected SMTP at least.
Also check if you have Conditional Access policy where you blocking legacy authentication.
And lastly if Microsoft shut down legacy authentication, you can still opt-out, but eventually you will have to use different smtp service like smtp2go or sendgrid. You can find here on how to opt-out.