Postfix sender_login_maps
As recommended you should add reject_sender_login_mismatch to your sender_restrictions to only allow users to send with their own address as FORM. Anyways some special cases are requiring you to login with one address, but send as another like WordPress on most php setups tries to. I created an account called relay@mydomain.de and changed some settings to allow this specific user to send with all addresses. main.cf sender_restrictions = reject_sender_login_mismatch,... master.cf...