Re: Fastest way to inject a lot of mail?

Aside of the recommended "Tuning" by Claus, your workflow the way I read felt inefficient. You prep the mail, fire up a sendmail MSA instance (just to wrap the message with proper mail headers), handle to another sendmail MTA or whatever (to add DKIM headers) and move it forward... if you already have a trusted MTA elsewhere, why don't you just deliver the message right into that MTA via the application itself?
If you are lazy (as I am), a rudimentary, poorly written, very insecure and extremely lazy bash script can do the job:
echo ${mail_message_complete_with_envelope_headers_and_ehlo} > /dev/tcp/$smtpsrv/$smtpport
Assuming you trust it enough to cut the authentication and StartTLS pieces to save precious CPU cycles. The DKIM header will still be added by the MTA, but by not spinning multiple MSAs which, combined with its dynamic libraries, you save quite some time to get messages moving.
Or if you really want to use sendmail as MSA because authentication/TLS/reasons, keep one running, and deliver the email message to it via TCP/IP. A fork() is always much faster than a full load.