Why this message was rejected?

[Marco Sulla]

One of my post on this list was rejected. The reason is:

> Blind carbon copies or other implicit destinations are not allowed.
>Try reposting your message by explicitly including the list address in
> the To: or Cc: fields.

I rechecked my mail and I added the user to the To: field, and the
python list address to the Cc field. I don't added any Bcc. I do not
understand why my message was blocked.