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.