title: Why would I choose simple over relaxed canonicalization for DKIM?
body: <p>DKIM supports two canonicalization schemes: relaxed and simple. The former is more lenient and allows for intermediary mailers to modify the email to a limited degree.</p>
  
  <p>The only data I could find is a <a href="http://mipassoc.org/pipermail/ietf-dkim/2011q2/016542.html" rel="noreferrer">survey of implementations</a> that shows the vast majority of email senders using relaxed canonicalization both for headers and body. (Noticeable fewer use relaxed for the body, but it's still a definite majority.)</p>
  
  <p>The DKIM specification says that all clients have to support both canonicalization forms if they support DKIM, so that doesn't seem like a major factor. Both schemes allow intermediaries to add headers. The only distinction I can tell is in the handling of the case of header names (not values) and the whitespace <em>within</em> a header. Given that, it seems like relaxed will always have at least as good deliverability, which is the aim of DKIM.</p>
  
  <p>(Of course, if I want to actually <em>sign</em> my emails to attest to their contents, I'd use S/MIME and certificates. DKIM is strictly about deliverability, right?)</p>