With the upcoming update with the Microsoft Online policy (Baseline) we're not allowed to use legacy authentication, as far as I am concerned this includes SMTP. Assuming we are not able to disable it, how will we be sending e-mail after October 2020 using C#, or other languages?
Modern authentication seems to become the one and only - preferred - way of sending e-mail.
In extend, how should one send email as/on behalf of a share mailbox with baseline enabled?
With the deprecation of basic authentication last October some stuff has changed. Basic Authentication for SMTP is still available but, as mentioned above, will eventually be deprecated as well.
I now use Microsoft Graph for a lot of things including sending email.
Please see these docs for more information on sending email using Microsoft Graph.