SpringFlower wrote:

I created a contact folder "F" with 300 contact info, including
myself and X. I send the mail to myself , cc to X, and bcc to "F".

Not possible. A folder cannot be an entry in a recipient field.

It was sent out succefully, however I received a system undeliverable
message of "503 BAD SEQUENCE OF COMMANDS" under each mail address of
the recepient, including me and X.

What version of Outlook? What type of account (SMTP, I'll assume from the
format of the error)? Do you have a mail scanner processing outgoing mail?
If so, disable it.
--
Brian Tillman