You can use SmarterMail 16x or 17x as an antispam gateway for domains hosted on O365, but since Microsoft has multiple ranges of IPs, you would need to do the following instead of setting this up as an Incoming Gateway/Domain Forwarding as you normally would...

1.) Log into your SmarterMail admin interface as the top level admin.

2.) Set up the domain in SmarterMail as you normally would. Do not worry. No mail will be actually delivered to this domain on the SmarterMail server. The domain just needs to be there for the next part of the configuration.

3.) Single-click on the domain to see its options.

4.) Scroll down and find the bottom left hand card titled "Email".

5.) You will see an option for Inbound Message Delivery. Click it and choose External (use host address).

6.) You will now see an option to add an external host address. Enter the MX record that Microsoft provided to you when you signed up for their O365 email service.

7.) Click Save.

8.) Log into your DNS server and point the MX record for the domain to the SmarterMail server. This will cause all incoming mail to go to the SmarterMail server first for antispam/antisvirus scanning before being sent out to the destination mailboxes on the O365 server.