First you have to make sure you setup the cron jobs after installing dolphin. You can find the instructions for the cron jobs here http://www.boonex.com/trac/dolphin/wiki/DetailedInstall

I have a virtual dedicated server and can send out 50 emails at a time without any problems. I will probably up that to 75 then 100.

Are you able to explain your problem in more detail?

I had some problems with the mass mailer after upgrading to 6.1.6 and was able to fix it.