I use Mailchimp to send out global emails to clients who have signed up for it. I do not add anyone to the list without consent.
Clients records such as email conversations and login details are held securely in Evernote and regularly deleted.
I use Paymo as my invoicing software and that keeps details such as email addresses.
I use analogue notebooks to write notes and ideas about work. None of these have anything that can be traced back to a client.
My computers are password protected.