If you're still reading, it's probably because you're trying to set up DKIM, or more specifically dkim-milter/dkim-filter on Postfix or Sendmail or something like it. I struggled with this issue on and off for a couple days. The biggest obstacle ended up being seeing logs like these in my maillog:
dkim-filter [...] SSL error:0906D06C:PEM routines:PEM_read_bio:no start line
dkim-filter [...] dkim_eom(): resource unavailable: PEM_read_bio_PrivateKey() failed
After a ton of searching online, the issue pointed to a formatting error in the certificate I created using a dkim-genkey.sh script in an article that explained how to set it up. Specifically, it created a certificate file that lacked the -----BEGIN CERTIFICATE----- style header/footer. I tried manually adding these back in, re-creating the line breaks to build a standard .pem formatted file, but none of these seemed to work.
Ultimately, the way I fixed it was to use the "dkim-genkey -d [domain name]" command that came with the dkim distribution to generate the key and DKIM DNS record.
After restarting dkim-milter and postfix, and sending a test message to check-auth@verifier.port25.com (domain names munged below, but otherwise this is cut/paste from my unix cli)...
$ telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mail.example.com ESMTP Postfix
MAIL FROM: khan@example.com
250 2.1.0 Ok
RCPT TO: check-auth@verifier.port25.com
250 2.1.5 Ok
DATA
354 End data with .
SUBJECT: Testing
Test
.
250 2.0.0 Ok: queued as 9A30E76C137
QUIT
221 2.0.0 Bye
Connection closed by foreign host.
Here's the response I got:
[snip]
----------------------------------------------------------
DKIM check details:
----------------------------------------------------------
Result: pass (matches From: khan@example.com)
ID(s) verified: header.d=example.com
Canonicalized Headers:
Subject:'20'Test'0D''0A'
To:'20'check-auth@verifier.port25.com'0D''0A'
Message-Id:'20'<20120411204224.3839576C138@www.example.com>'0D''0A'
Date:'20'Wed,'20'11'20'Apr'20'2012'20'13:42:13'20'-0700'20'(PDT)'0D''0A'
From:'20'khan@example.com'0D''0A'
DKIM-Signature:'20'v=1;'20'a=rsa-sha1;'20'c=simple/simple;'20'd=example.com;'20's=default;'0D''0A'
'09't=1334176963;'20'bh=pN6k1Y/YJkrV9QxvE85uD6qiAqw=;'0D''0A'
'09'h=Subject:To:Message-Id:Date:From;'0D''0A'
'09'b=
Canonicalized Body:
Testing'0D''0A'
DNS record(s):
default._domainkey.example.com. 1800 IN TXT "v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDX3yuDnEl5XL2FPPKxW5h9iW43ZMxlg2hI/23BiYXczMFokP7AjsNMZKzg/9s1E2TswBFMFnM26ZDQtV7P3FmlcRO2H3YmRfIOftY 93+c88KXKN1fET5PfyKjWqraYHJiNID+Vwzn5njDfI1WnDxkzVPEx/30v7W3tfbCpAJGhkwIDAQAB"
