IOS 13.4.1 I pad

In settings I can only set ‘ you can receive messages aNd reply from the following address.. I put my principle e mail address here.

I went to my address book and found another e mail address of mine, but for some reason I cannot figure out how to add it to my I message addresses.

When I type it in, in I message as if to send a message,the e mail address turns red, and the message will not send. Any message.not just a forwarded message.

Nothing about forwarding either on my I pad or otherwise.

Any suggestions?