possible DKIM signing problem with a lot of recipients?

[TobiasP]

Debian 8.7‬
Plesk Onyx Version 17.0.17 Update #18

When DKIM signing is enabled for a domain an I send an email to e.g. 5 recipients the email gets 5 DKIM Headers added:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=...;
s=default; t=1489450241;
bh=...; l=6;
h=From:To:Subject;
b=...
(replaced some content with ...)

is this a bug or is it the way it is intended?
the problem is that if someone sends a mail to lets say 25+ people the email header get so big that certain email providers will reject the email with "header size exceeds maximum permitted"

Maybe it does happen because the "to" is signed (look at h= setting)

for comparison:
office365 signs these headers:
h=From: Date:Subject:Message-ID:Content-Type:MIME-Version;

they don't sign "to", maybe because it causes issues with many recipients...

Edit:
in plesk onyx dkim signing is handled by the milter running on 127.0.0.1:12768 right?
how can I change which headers get signed?
@plesk developers: maybe you should consider removing the "to" header from dkim signing default setting

Cheers,
Tobias

 

[AYamshanov]

Hi Tobias,

It is the same as "315 duplicate DKIM Signatures in one Email Header". Submitted as PPPM-5476.

 

[TobiasP]

Hi Tobias,

It is the same as "315 duplicate DKIM Signatures in one Email Header". Submitted as PPPM-5476.

oh sorry for double posting then.