Report on Communication Preferences by Client
There are communication preferences for each client (e.g. Do not contact by email, do not contact by phone, etc.). Allow me to run a report that includes this information, so that if I want to send a mass email to clients I know who to exclude.

The client export list has been updated to include the client’s communication preferences! When navigating to Organization → Export Data, you’ll be able to view which clients have opted out of text, email, or phone communications so you can quickly see which clients prefer to be contacted by a certain method.
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Steve Johnson commented
If it's too hard to create a report then another easy solution would be to add the "do not contact by email" flag as a new column on the "export all clients" tool. We could then sort that Excel file and only grab emails for clients that have agreed to be emailed.