This behavior is intended. When we worked on extending folder sharing so that it would work over protocols, we evaluated the behavior of sharing in Exchange and Office365 accounts. When possible, we made sure that SmarterMail's behavior was similar to those systems and also that the behavior was as consistent as possible across web mail and protocols.

"ReadOnly" means that you can "read" an email, so I think it is correct that after you read it, the email status becomes "read"...

I see what you guys mean, but I would argue that as 'read only' you are not the owner of the email and only the owner or manager should be able to change its status. Otherwise I go log into the account as the owner, and having my filter turned on for "unread" messages and I never see it.