Post your domain and the affected IP addresses and we can test.
Sounds like you may have multiple issues with your configuration and security (possibly an open relay), and may also have some high-volume spam originating on your SmarterMail server.
There are some very strict rules that both you, and your hosted customers, need to abide by, if you are going to run a mail server these days. Failure to properly configure your server, lock down your mail server, scan for inbound and outbound spam, setup rDNS and SPF records setup DomainKeys, DKIM, DMARC and feedback loops will continue to get you blacklisted.
So post your domain name and the affected IP addresses.
Once we have your information, we can check your domain name and IP addresses, to see if our external scanning tools can reveal anything you haven't found.
If, after I respond with the test results, you and the support people at SmarterTools cannot resolve the issues, ask for help from someone you feel comfortable with and take it from there.
Mail servers are not "plug-and-play," but require a lot of detail to attention on both the setup and maintenance side. SmarterTools makes the process a whole lot easier with their SmarterMail product.
I still have a couple of clients who have "mixed" MX server environments: Trust me: working with SmarterMail is a WHOLE lot easier than working with Exchange - been there, done that, have the t-shirt.
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting