Christian Rößner:
Unfortunately I found out that always the very first header of an
earlier milter is not visible in my milter.
christian,
The milter API knows two ways to add header:
1) https://www.milter.org/developers/api/smfi_addheader
2) https://www.milter.org/developers/api/smfi_insheader
insheader allow to specify a position. Position 0 is special: "Topmost".
I found that many milter use position 1, other use 0.
With milters calling insheader and hdridx=1 I never saw the problem
you described.
Andreas