Communication between any email client and an email server is governed by protocols. Whether it’s IMAP, POP, MAPI/EWS, EAS or WebDAV every feature you have access to in the client is dictated and handled by a protocol. That said, not every protocol contains properties that have a one-to-one relationship to the mail server. Conversely, not every feature in an email server will have a complementary property in an email client.
For example, Tasks in Microsoft Outlook. Tasks in Outlook are handled as “To Dos” in the Outlook client. When syncing Outlook Tasks to SmarterMail using a protocol such as EAS, these "To Dos" don't sync because EAS doesn't offer that property when it syncs to a mail server. The same is true when you flag a message in Outlook as this also shows up as a “To Do”. In SmarterMail, flagged messages WILL show up as flagged messages, but there’s no “To Do” reminder set as that property doesn’t exist.
User Groups in SmarterMail are a good example from the mail server side. User Groups allow you to create Read Only groups of users, and then that group can be used when sharing within SmarterMail. This makes it easier to sharing items to multiple different individuals without having to share them one-by-one. User Groups only sync to Apple's Contacts when using CardDAV as CardDAV is the only protocol that has a property associated with that feature. EAS and MAPI/EWS do NOT use that property.
There are multiple examples of these types of exceptions for practically every syncing protocol available. SmarterTools does what it can to accommodate every feature available to users, regardless of the email client they're using. However, due to protocol limitations, some things simply can't be shared, from both the mail server side as well as the client side.