(add-hook 'mu4e-compose-pre-hook
          (defun my-set-from-address ()
            "Set the From address based on the To address of the original."
            (let ((msg mu4e-compose-parent-message) ;; msg is shorter...
                  (setq user-mail-address
                        (cond
                         ((mu4e-contact-field-matches msg :to "mail@christianlask.de")
                          "mail@christianlask.de")
                         ((mu4e-contact-field-matches msg :to "laskc@smail.uni-koeln.de")
                          "laskc@smail.uni-koeln.de")
                         ((mu4e-contact-field-matches msg :to "business@christianlask.de")
                          "business@christianlask.de")
                         (t "mail@elfsechsundzwanzig.de")))))))