Coldfusion 9 Fails to send mail when using cfMailParam to set the content type to text/html

We recently upgraded to ColdFusion 9 and have had our share of migration pains. Today I discovered a new one. We have a newletter that goes out on a fairly irregular basis and today was the first one since the migration. Hours after it was scheduled to be released I got notified that no-one has received it yet. So I check the undelivered mail and yup, there are almost 4k messages that ColdFusion refused to send to the mail server.

As Sherlock would say, the game is afoot. First, let's rule out the most likely suspects. Looking at the mail log, I see errors for them all, but the error message is blank. I check the mail server connection, it's fine. I try dropping a few back into the spool, but they just get kicked back to undeliverable. I check the SVN logs and see that no code changes have been made to the file involved. Great, not going to be an quick and easy bug hunt.

Interestingly, while looking at the undeliverables, I was still able to see messages hitting the spool and processing normally. Thinking to myself, "what makes them different" I manage to open one before it processes out and start to compare it to the ones that won't. Fortunately the difference was on line one: the ones that won't sent say "type: text/html" the ones that will say "type: text/html; charset=UTF-8". So I change that line on one of the failures, send it back over to the spool, and it processes normally. Great! Fix it in all of them, send the batch over to the spool and we are in business.

But why did this happen?

I pull up the code for the mail that went through fine, and for the one that didn't and look at the cfMail tag. and here is the big difference:

Good:

  <cfMail [typical settings] type="html">
    Mail Message
  </cfmail>

Bad:

  <cfMail [typical settings] [no type attribute]>
    <cfmailparam name="content-type" value="text/html">
    Mail Message
  </cfmail>

The bad code here worked fine in CF7, but just results in undeliverable mail in CF9. I can't tell you why, but hopefully my investigations help save someone out there some time.