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I' ve configured MailSystem and HTMLMail so that my "Site-wide default MailSystemInterface class" and "HTML Mail module class"
are both "HTMLMailSystem__SmtpMailSystem".
However, if I do a cache clear or just leave the mail system settings page admin/config/system/mailsystem the HTML Mail module class reverts back to HTMLMailSystem and I cannot send mail. So long as I stay on the page or do not clear the cache its possible to send mail.