You were BCC'd on those messages. Sender sent the message to
himself and BCC'd you.

What about the ones received that say Undisclosed in the To box?

Same thing without the 'Sent the message to himself' part. (ie, the
To line was blank)

Tried it but I do not get a message showing To: undisclosed
recipients

It depends on the sending client and mail servers, IIRC. What is it
you want to do?

Send emails with undisclosed redipients in the to line

Just leave it blank and let whatever happens happen.

Or do a mail merge so people get their own address in the To line.
--
f.h.
Microsoft Outlook MVP