Creating new Mail messages without the blind carbon copy (BCC) field is annoying. The issue has been discussed here on MacOSXHints before. I had submitted this issue to Apple back in April, and I guess that we were not the only ones complaining about this because the next version of Mail will allow, via Preferences, to set the BCC field to display permanently. But that doesn't help today...
Meanwhile, I tried many many different approaches (like forcing all CC to be BCC or setting a default BCC to myself to force the field to show) but this is so far the best way:
tell application "Mail" set newMessage to make new outgoing message tell newMessage set visible to true make new bcc recipient at end of bcc recipients end tell activate end tell
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030809041730354