If you set up OpenSSH on the AIX server you will be able to connect to it from pretty much any platform using an SSH client like the already mentioned PuTTy or Cygwin in Windows.
OpenSSL is a set of libraries that provide the Secure Sockets Layer and also TLS, Transport Layer Security. OpenSSL can be utilized by other programs (FTP clients and server, etc) - if they support it - to provide secure communication between the server and the client.
There are several email servers that support encryption using SSL/TLS. If you run Exim, here
is the relevant part of the documentation and here
is the relevant part from the FAQ. Most modern email clients support encryption, among those are Mozilla Thunderbird, Pine and Sylpheed.