We currently have the following settings: PCNTL enabled, 5 campaigns in parallel, 10 subscriber batches in parallel Subscribers at once: 10,000 and we just finished a campaign to 275K subscribers in 1 hour 46 min on a 4core server with 4GB of RAM. During the send the CPU was around 50% utilised. In our environment we have MailWizz queue into a local postfix which then relays into Port25. When tailing the local maillog, there are certainly pauses and I do feel that I should be able to push the campaign out much faster into the local postfix. Our Port25 manages the QoS and sending limits/backoffs across a number of VMTAs and I just want to have the campaign pushed out from the local MailWizz server as fast as possible. If anyone has some stats/config suggestions, this would be appreciated - especially throughput examples. In comparison, our current Interspire manages the same campaign in under 1 hour. Correction: The above settings did not result in 275K mails being sent. MailWizz reported this, but when looking at the postfix maillog, only 38K mails were processed. I have logged a support ticket, but we have run out of ideas - in my opinion the issue is possibly related to PCNTL. In another test with PCNTL, the MailWizz log shows 52593 mails logged, Postfix has only received and processed 3988. When turning PCNTL off and using default settings, it takes 5 hours for 73,000 mails into local Postfix.