speed DKIM signing in mail server

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http://serverfault.com – I'm running email sending campaigns with milions of emails, and I need to sign each and one of them with DKIM. Unfortunately that's the bottleneck of the process. I wonder if I can calculate a DKIM signature ONCE and use it in all of the mails (maybe only signing the subject, that is common to the whole campaign). Is that possible? (HowTos)