new mail to

What does “x” mean (to the right) when I add an address in the To field of a msg?

When you start typing in an address, eM Client will suggest recipients from your Contacts and from previous recipients, who may not be in your Contacts, but are stored in your Recipient History.

Clicking on the x will remove that data from the Recipient History.

You can find the Recipient History in Menu > Tools > Settings > Mail > Compose > Recipient History.

How do I distinguish addresses from history and address book? An “x”?

If an address is not present in my address book I get an option to add it to my address book when right click.
If I do that again, I still got an option to add it to my address book, not an option to edit it?

Yes. If the address is in your Recipient History, it will have an x.