There was one case when FBI needed to get into iPhone of a criminal. So they hired a professional hackers which successfully did that. Apple has a strong security, but it can be still hacked.
Anything and anyone can be hacked. The Pentagon and even the I.R.S. has been hacked. There is no such thing as 100% security in this high tech world anymore.
An in depth post on secure email providers was recently published here: https://restoreprivacy.com/secure-email/