Sendmail breaks lines in the body at 990 characters. Since this happens after signing, the signature will always break.
You can find documentation about this online, as well as ways to work around it, such as:
http://lists.roaringpenguin.com/pipermail/mimedefang/2003-September/017418.html
In any case, there are no line limit constraints in opendkim.
From: opendkim-users-bounce_at_lists.opendkim.org [mailto:opendkim-users-bounce_at_lists.opendkim.org] On Behalf Of vfx9as
Sent: Friday, November 19, 2010 8:53 PM
To: opendkim-users_at_lists.opendkim.org
Subject: Re: An error in the text line length
(10/11/20 12:31), Murray S. Kucherawy wrote:
> Can we see a gzipped copy of your sample message?
The 692 was a mistake.
Ok 989 characters
$ printf "From:root\n\n%0989s" 989 | sendmail root
99 characters error
$ printf "From:root\n\n%0990s" 990 | sendmail root
Received on Sat Nov 20 2010 - 05:51:30 PST