As Ben stated, ideally you would want your users to utilize Remote POP to retrieve email rather than using a Forwarder. Forwarding has been strongly discouraged by IETF RFC since 1994 and has been fundamentally broken since April 2014 with the widespread adoption of DMARC (Domain-based Message Authentication, Reporting & Conformance) policies. Any email from a domain that has a DMARC policy published in DNS will not be accepted upon forwarding, resulting in emails from major vendors such as Facebook or PayPal never being received from a Forwarder.
That said, if you set SmarterMail to not forward emails marked Low Spam Probability or higher, and do basic Anti-Spam tests on Outgoing SMTP then you are doing enough due diligence to keep from taking a ding on your reputation. (A lot of Spam might not be caught when it comes in but by the time it goes to be forwarded out it usually gets caught by the very same RBLs & URIBLs just a second or two later.)
You may also want to signup for Feedback Loops with all the major providers. For GMail specifically you'll have to authenticate your domain(s) with Google Postmaster Tools. This will help keep your reputation positive as there will be an abuse contact that these vendors can send complaints to.
Lastly, once you get a positive reputation and keep it positive for over a year most dings won't actually hurt your reputation. Even a compromised account sending out a large volume of Spam won't ever hurt it as long as you stop it within 24 hours. To go from a long-term Positive to a Neutral or even a Negative score requires a certain level of neglect and apathy from the MailAdmin.