In order to use outside tools with other people using ProtonMail it appears you need the ProtonMail bridge.
What is the ProtonMail IMAP/SMTP Bridge?
In a single sentence, ProtonMail Bridge is an application that allows you to use your ProtonMail encrypted email account with your favorite desktop email client such as Thunderbird, Apple Mail, or Outlook, while simultaneously retaining the zero-access encryption and end-to-end encryption that ProtonMail provides. The best part is that this does not require modifying your email client or making changes to your existing workflow. Use email like you have always used it, and the Bridge will automatically encrypt and decrypt messages in the background