When setting up an IMAP account in MacOS, you're asked which items you want to sync. Two of those options are Email and Notes. If you decide to sync Notes, it's important to understand that IMAP notes behave a bit differently than notes synced via EWS, and that this behavior occurs in BOTH SmarterMail and Microsoft Exchange.

First off, IMAP creates a new folder in the Notes app on MacOS specifically for the IMAP account. However, any existing notes in other folders within the Notes app are not synced. 

Next, any notes created in the Notes app on MacOS sync back to an email folder that is created in SmarterMail. They do not sync to the Notes area of SmarterMail. Instead, a new folder is created that sits with other email folders, such as your Inbox, Junk Email, or any custom folders you've created.

Lastly, it appears that the synchronization between SmarterMail and the MacOS Notes app is one-way: that is, it's a sync from the MacOS Notes app to SmarterMail -  not vice versa. Therefore, any note created in SmarterMail is not synced back to the Notes app. 

As near as we can tell from this, since Exchange internally treats its Notes folder as an email folder, Apple has decided to do the same. SmarterMail, on the other hand, treats Notes and email folders separately. However, since Apple "sees" notes as an email folder, thanks to Microsoft's implementation, MacOS notes sync to SmarterMail as an email folder.