There are multiple filter options available in your client management. Most of them don't need any description, as the title tells you what you can filter for.
Use the search field drop down to search for multiple fields in your client manager. You can even search for entries in your user defined fields.
In front of the client names are checkboxes. There you can mark multiple users to create a selection or a mailing list.
Right click on your selection and save it or create a mailing list.
In the next step you can filter the clients for the selection you've just set in your search area. Open the "selection" menu and choose the selection you've just saved.
You can create global client categories to filter your clients accordingly. For example you can create a category: Christmas card and then filter for all clients who should receive a Christmas card. Add new categories in the client data.
In the client manager you can only see the categories available and checked, but you cannot edit the data there. You have to double click to open the client data.
In the next step you can filter your clients using the search area.
Another good option is to group by the category. You'll get a result with all your clients, grouped by category.