Looking at the manual for sendmail, there seem to be more flags that would also have to be ignore:

-Am
-Ac
-B
-bd
-bh
-bH
-bi
-bl
-bm
-bp
-bs
-bv
-C
-C
-F
-f -> Currently supported by mhsendmail
-G
-h
-I
-i -> This flag already exists for sendmail compatibility.
-L
-m
-N
-n
-oA
-O
-o7
-o8
-oi
-om
-o -> This flag already exists for sendmail compatibility.
-r
-R
-q
-qI
-qR
-qS
-t -> This flag already exists for sendmail compatibility.
-U
-V
-XV
-v -> Currently supported by mhsendmail
-X

Some of these flag also have a value, sometime without a space to separate the value.

I think the most sane approach would be to simply ignore any parameter other than those supported...

However, if that isn't desirable (as that would break with the current working of this library), I don't mind adding a pull-request that adds these parameters with the "Ignored. This flag exists for sendmail compatibility." message...

@ian-kent Can you please comment which approach would better suite you?
A. Adding every single parameter with ignore message
B. Changing parameter handling to use a white-list