It's possible to quickly convert a job list to a client list. This can be useful if you need to view other information about the clients in the job list, or if you wanted to send them a bulk email.
Open your jobs list from Work > All jobs.
Apply any filters to the list you need, such as job filters and status filters.
Select all of the jobs shown in the list.
Choose View as client list from the Actions menu.
You will then be shown a client list with the appropriate clients.
From here you can then bulk email these clients.