>>Why do you remove ARC signatures ?

Because they have no relevance when you have rewritten both MAIL FROM, RCPT
TO, and MIME From, and MIME To.
If the mail is forwarded again, then both second forwarding server and final
server will test against YOUR server, so it's the second forwarding server
that in that case should add ARC signatures.

But forwarding in 2 step is uncommon and useless, better to aim both
forwarding addresses to the same final address.

>>That might help the message reach the mailbox, but the recipient will
>>have to look harder to see who the message claims to have been
>>originally sent by.

Agreed with that. The info is in "Reply-To" instead.
It's a tricky thing, because writing 

From: "original_sen...@server.com" <forwarded_u...@yourserver.com>

may trip anti-phishing / anti-spam systems.

If you absolutely must preserve sender information, encapsulate the mail in
a new message/rfc822 object instead...

So a mail that looks like this (if forwarded_user is a forward to
exam...@gmail.com):

From: original_sen...@server.com
To: forwarded_u...@yourserver.com
Subject: Hey
Content-Type: text/plain

Is sent as:

From: forwarded_u...@yourserver.com
To: exam...@gmail.com
Subject: Fwd: Hey
Content-Type: message/rfc822; boundary=1234

        Encapsulated email follows
        --1234
        From: original_sen...@server.com
        To: forwarded_u...@yourserver.com
        Subject: Hey
        Content-Type: text/plain
        --1234--


>>Note. This example is confusing

Oooh sorry about that.
I tried to be clear with youru...@server.com being the sender,
forwarded_u...@example.com being the address with a forward configured, and
somewh...@gmail.com being the forward target.

Replaced youru...@server.com as original_sen...@server.com instead.


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