Hello Dexter,
You can encrypt all outgoing messages by just four steps configuration. Here are the steps:
Step 1. Open Outlook, Click on File then click on Option
Step 2. Go to Trust Center → Trust Center Settings
Step 3. Under Encrypted email option check Encrypt contents and attachments for outgoing messages
Step 4. Now, send any email message with attachments in encrypted form.
Feel Free for any more query!
Have a nice day!