@frm.mwz / @VVT - The limit in mailwizz is 1 by default, so it's exactly what happens now, one message per connection. You can now adjust this for any smtp server from backend, and change from 1 to whatever your server supports. Swiftmailer doesn't have a keep alive feature unfortunately, so i am not sure how we will go about it. We don't destroy the connection for swiftmailer either, so if swiftmailer doesn't close it after sending we should be good, but i think it does. If @VVT can run a few tests with swift as well, this would help.