If I put X in "To: " then Y and Z in "Bcc: ", Y and Z will see that I sent the email to X, not to them. I want them to see that I sent the email to them (Y sees To: Y, and Z sees To: Z). Is is possible ?
BCC is the only way to accomplish this unless you write a script to loop through a list of recipients and send each the same message using their name in the "To: "
Don't use a "To:", just BCC everyone. Also, this doesn't truly hide the addresses. If anyone is smart enough to show the full header of the message, they can see all the recipients.
What is your spec? Are you trying to spam people? Are you trying to avoid generating that much traffic so your mail server won't get blacklisted due to spam by sending the single message with all the recipient names?
If not, you can use any one of many different scripting languages (ie php, ColdFusion, ASP) to loop through a mail list or a query of e-mail addresses and send an individually packaged/wrapped e-mail message with only one "To: " to each recipient.