Hi Steve,
Double checking I'm reading this right. You have mailing lists / newsletters, but currently do not maintain your own email servers, and it's a 3rd party you're using to send the email?
Is there any problem that you are trying to solve or are you trying to make sure that a work services continues to be a working service?
If you are not experiencing any issues why make a service change at all? Just curious, who are you using now that you are not having a problem with?
There is a lot that goes into the reputation of your email. Some starter tips can be found here -
http://www.emailquestions.com/email-articles/2437-top-10-email-marketing-tips.html. Beyond those tips todays experts are beginning to say that it also matters how often your subscribers are clicking the links or taking some other action based on your email besides deleting it.
What list of services are some of the companies claiming to provide that you are looking to have "manage your email reputation"?
My general advice is that if you are not comfortable running your own mail servers, using one of the services within the link above such as Mail Chimp or Contact Contact sounds like they would be a better and cheaper alternative. $900/mo for the amount of email that you are sending is too much, check out some more services and get some more quotes.
to Email Questions!