To and Cc appear as you composed them. Before, the outgoing message listed every SMTP recipient in the To header and dropped Cc, so a copy sent to several people showed all of them (a Bcc included) in To. Now each recipient sees your To and Cc headers, every recipient still receives the message, and an address that appears only in the SMTP envelope (a Bcc) is never named in any header. A message sent only to Bcc recipients is shown as To: undisclosed-recipients:;.
List and custom headers are kept.List-Unsubscribe, List-Unsubscribe-Post, Precedence, Auto-Submitted and your own X- headers now reach the recipient. Gmail and Yahoo's one-click unsubscribe rules need the first two. The same headers are refused as over the HTTP API (From, To, Cc, Bcc, Subject, Date, Message-ID, DKIM-Signature, Received and the authentication headers, and anything starting X-Motorical); a value longer than 998 characters or with control characters is dropped, and at most 20 are kept.
Nothing changes in who receives the message or who it is from.