Syncing among devices is okay -- partially

Messages and emails that I receive sync fine among my Mac, iPhone, and iPad, but the deletes do not always, meaning that to delete items I must (usually) do the deletes from all three. It's like Messages and email are using IMAP for the items received, but acting like POP for deletes. I said "usually" because sometimes the deletes sync maybe half the time. Deletes tend to work better if everything is on the same Wi-Fi but if they are not, deletes might not sync.

Any ideas?

All devices are kept up to date with their operating systems.