Tune interspire email server

We have been running interspire email marketer for many years to send our paid, opt-in daily newsletters. Recently (about a month ago) we experienced intermittent issues with the send slowing down. The problems only occur when we connect via SMTP to a third party server - but it does it with any SMTP third party I believe.

I need someone who is experienced with this software to help debug the issue and also advise on the best way to optimise the sending speed.