i import mail from gmail and gmx, i want all sent mail to go with the gmx address. how can i do that?

Simply set your gmx as default account in the account settings.

that doesn’t do it. i set gmx as default and at the top of each email i compose i am still offered which email account to send from, the default is the address i received the mail from.

In that case disable SMTP service in every other accounts. It should help.