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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?

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Yes this is possible but defeats the reason of using DKIM. It is intended to sign each of the mails separately! Otherwise I can sign mails that look like your mails. With any content I like to use as long as it has the same subject.

If you need more speed, buy new hardware. What about a computing cluster? Or drop DKIM at all as you seem not to care about signing your mails.

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