StrongWebMail claims to offer "the most secure email accounts on the planet." Their claim is based on a callback verification system that works like this: You use a typical username and password to login, but before your login is allowed "\[the account owner\] must receive a verification call on his pre-registered phone number."
But as with all third-party online services there's a weak spot in the armor: You have to trust the service provider not to be snooping around in your data.
So what StrongWebMail really offers is defense against most intruders accessing your private correspondence. That still leaves room for plenty of other people to snoop, including potentially non-trustworthy employees of StrongWebMail and communication backbone providers that your communication might traverse.
The bottom is that the same principle applies today just like it did 15 years ago: Nothing you send across the Internet is truly private.