My members have been complaining about not receiving emails from my IPS site.  I found out that my host is filtering a lot of the emails that are generated via IPS as spam. I've already done all the server site and DNS settings for DMARC, DKIM, SPF, etc. 

They are recommending that I change the subjects of the emails that are being generated. Is there an easy way to add a prefix or something to all outgoing emails?

There is no way, outside of modification, that you can add a prefix to all outgoing email unfortunately. 

I've been of the opinion for years that this should be customizable.