I tried to send email to someone on my secure friends list and CC'ed ( carbon copied ) the message to someone not on my friends list and Outlook reported the following error :
"SecExMail rejected the message because you mixed plain text and cipher text recipients."
Consider the following scenario. You compose a message and address it to two friends, Bob and Alice. Bob has a SecExMail key - Alice does not. However, your email client sends one message only. Your mail server is responsible for distributing your single message to all recipients, not your email client. This is a design feature of SMTP, the protocol which governs distribution of email messages across the public internet. Because the message has been encrypted however, Alice cannot read it and asks you to resend the same message again, in plain text. You oblige her.
If someone were to observe you sending the same message twice to the same recipient, once in cipher text and once in plain text, they could reasonably mount what is called a known plain text attack on the session key or your SecExMail key because they know both the input ( plain text ) and the output ( cipher text ) and might therefore deduce your key. For this reason, SecExMail has disallowed the action.
Simply compose two messages, one to the person on your friends list and another, separate message to the person who is not on your friends list.