We had a lengthy discussion on the postfix-users mailinglist about this:
My initial problem with dkim-milter was this:
Jul 25 11:26:10 mail-ausfall dkim-filter: B93813DC16: dkim_eoh(): internal error from libdkim: header too large (max 4097)
and looking at the mail I see a user adding a shitload of E-Mail addresses to the To:/CC: headers, instead of using the Bcc: headers.
My users are surely not dumber than the average user out there.
I guess other users at other companies will also send mail with large lists of recipients. Their mail will fail as well.
Vicot Duchovni notes:
milter must support folded headers of up to 32k in length (Sendmail limits the total header size to this value, so headers larger than 32k are problematic already). Since milters are still primarily for Sendmail, the 32k size limit is natural. The next natural limit is 64k, which is the milter protocol packet size limit, this would accomodate possibly larger future header size limits in Sendmail.