One very useful feature would be to add up to date filtering systems in report only mode (eg SpamAssassin), SPF / DKIM / DMARC checks, etc. Without having your own up to date inbound MTA, it's very difficult to test your own mails for things that might be triggering spam filters.
Do those services really have a feature like that? That seems like they wouldn't people to easily figure out if an email the have sent makes it through filters. Maybe I'm wrong, I have never had to worry about email in a business environment.
SpamAssassin, DKIM, SPF, DMARC are all things that you can check locally, so yeah, you can def. determine if an email is going to get through at least basic filters. Note that SpamAssassin is more a framework, and that a lot of this stuff is going to be bayesian/machine learning kinda stuff, so while you may get through basic level filters, people pay a lot for the higher end stuff, and you can't test those kinds of things. (barracuda email filtering and that kind of thing)
