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.