[Exim] Return code for "there is no valid sender in any head…

Hello,

I am currently having a problem with rejected mails due to "there is no
valid sender in any header line". The problem is that the other system
makes a retry. So far, so good, as explained in the manual. But: The
admin claims that he sees a 451 return code! Is there any way why exim
3.16 should return 451 instead of 550 for this error message? I failed
to reproduce this so far, but I asked for a session log to prove it.

I would appreciate if exim would log the error codes as well as the
message just to be sure what exactly it said to the other side.

Michael