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Here's my observation and this has been an observation since using smartermail for a few years.
1) Overtime, all of a sudden some people who send emails without a subject "FAIL DKIM".
2) Sometime, well almost all times, when a new version is installed "The issue goes away" and DKIM no longer fails with a blank subject line, then overtime the issue comes back.
3) When the blank subject occurs, Smartermail actually changes changes the content of the message causing all other DKIM validations to fail with it.
Why does this happen, I have no idea - But I'm bringing it to your attention. Hopefully, someone will be able to look into it and find a resolution to this issue.