My application was rejected. What can my host/sponsor in Germany do?
Your host/sponsor can for example send a supporting letter, which you can add to your letter reasoning for the issuance of your visa. He/she can not, however, personally request for the reasons for the rejection at the Embassy. If the sponsor/host wants to do this for you, he/she requires an authorization in writing from you, which he/she then submits to the Embassy, together with his/her letter of remonstration.