Help! Message must be sent by Unicode?

[Jeffrey Jolando Sim]

Title: Help! Message must be sent by Unicode?

I don't know what I did lately but suddenly messages I am about to send out, there is a dialog box that says, "This message must be send as Unicode which may not work with older mail client. Do you want to sent the message in Unicode?".  
My corporate PC user cannot receive my message properly, especially with attachment. They cannot open my attachment or it turned into garbage!
what must I do? The last thing I can recall doing any changes was that I changed from sending html to plain text format in the preference. However,  I had it change back to html format again. I wonder if this the cause of the problem? Regards, Jolando