I'm using Apple Mail and IMAP to get my mail from Zoho. Everything works great except one thing. When I flag a message in Apple Mail with a flag other than red (i.e. green, blue etc.) the flag color changes back to red after a while. Usually after the next synchronization. 

Curiously, this doesn't happen with my GMail account which I also added as IMAP to Apple Mail. I can flag a message with blue, green etc. flag and it never changes. 

Do you have any ideas why that is happening? 

Thanks.