It’s not going to work the same.
One of the go-to methods for contacting many clients is mass-email.
Here we use Campaign Monitor, but there are many others including the venerable MailChimp.
These things are all going to break with the way Apple is changing the way it connects to email services.
This is to give your clients (and you) more privacy. So, it is a good thing.
It will impact your mass emails.
Here is a great video from Campaign Monitor, detailing the changes.