Not sure why this was downvoted. If you're in an outlook/exchange integrated environment, you only have to click "encrypt/sign this". It's almost transparent to the user and in addition it would raise an error for the original sender. (saying the key for user [email protected] cannot be found)

And how would it help to automatically encrypt internal e-mails when an e-mail accidentally gets an external delivery address because someone fat-fingers?

You can set up the encryption so that it requires a verified public key. If you mistype an address, you won't have their public key, and can't encrypt the message to them.

Do you understand how PGP encryption functions?

Yes, I understand (on a superficial "don't actually use it on a day-to-day basis, but have encrypted e-mails before" level)

I am imagining this program that looks through your To: field for recipients in your address book that have shared their public key with you. If it finds any, those people get a copy sent to them which is encrypted and only they can read...

Now what happens if you accidentally try to send mail to someone not in your address book?

I guess hopefully you get a big popup that says "Warning: mail will not be encrypted!" Maybe not?

I've used the command-line tools to encrypt files and send encrypted attachments. It's nothing like "automatic" and it's certainly not an envelope for the whole message.