Q: ARE ALL ICLOUD EMAIL ADDRESSES SAFE FOR IMPORTANT EMAILS?
A: Apple email sends and receives using encrypted protocols. However, you have no certainty if encryption is present from the point between Apple and the other person. Because of this, you cannot consider Apple email secure. The same is true for almost every other email service.
Therefore, with very few exceptions all email is insecure.
The solution is to encrypt your email end to end. There are several options to do this, the most common being: PGP/GPG, S/MIME, and now Virtru.
You could also use an email provider that is built from the ground up for security. One of the better is