[Djigzo users] Mail trouble

He everybody, last week suddenly my gateway stopped delivering emails only to extern, our Exchange told me this error: XXXXX.XXX.XX #503 5.5.1 Error: need MAIL command ## Then I created a new one with the backup settings of it and everything works fine again. Today I try to find this error in the log files and did not see it on the gateway, also a test mail works fine. Any other person who has experience with that? Thank you. Marc