e-newsletter does not follow setup for sending emails

hi there,
today we wanted to send a newletter to 2000+ recipients with setzup 400 Mails/h and paused by 10 sec. We found out, that whatever setup we choose, all 3...4 seconds a new mail is handed over to our mailserver - wheter we set 10, 20 or 30 sec pause between its 3..4 sec only. We tried to delete the hourly limit, that rised the break up to 7..10 sec - unless we set up 10, 20 or 30 sec. So still its not really close to the setting. This scenario happens for manual sending as well as for sending via cronjob.

I am happy for help - since we want to avoid being listed as spammer - google's and microsoft's domainservice are very fast in blocking..

thx, Rob