Linux – Postfix – how to add emails to the queue without being delivered

That sounds normal. If you run multiple connections in parallel submitting mail, do you still get 90ms per mail? This will let you easily raise your rate that you can submit mail.

I know that other MTAs (sendmail) will fsync() to ensure that the accepted mail is committed to disk before sending back the 250 response, so that email doesn't get dropped if the system crashes after accepting the mail. postfix probably does as well.

You could test this by putting the mail spool directory onto tmpfs temporarily and seeing how long it takes to accept mail.

Other things that it could be doing during that 90ms window are: virus scanning, blacklist checking, etc. But you'd probably know if they were configured.

This is just an example and should guide you in the right direction. I haven't actually tried it before posting.

In master.cf comment out the line containing bounce in the first column. Then create new lines with something like

bounce   unix  -       n       n       -       -       pipe
  flags=DRhu user=vmail:vmail argv=/usr/local/bin/deliver -f ${sender} -d ${recipient}

This will pipe all bounced mails to the script /var/local/bin/deliver with some parameters. The script is executed as user "vmail" and group "vmail" (change accordingly). This script then needs to dump stdin to a (random) file and location. Probably you can use already existent scripts like procmail.

It will not disallow outgoing mails but "catch" the bounces you are after.