When sending a new email, in the subject box enter the word encrypt in squared brackets before typing the subject of your email, for example [encrypt] Test Email.

Once you have completed your email, click send as normal.

The recipient of the email will receive the below message: -

The recipient will need to click on words:- Read the message.  They will then be presented with the following screen:-

The recipient can then select one of the options, if you have sent the email to a Microsoft account such as hotmail.co.uk, outlook.com or they have office 365, then the recipient can sign in using these accounts to view and reply to the email.  The option for most users will be to click sign in with a one-time passcode. When the recipient selects this option, they will be emailed a passcode as below:-

The recipient can then enter this code into the one-time passcode box as below:-

The email will then be displayed to the recipient in the web browser along with options to reply as below:-