[haus]

Huh, I never noticed that. You're right, the only icons appear to be unread and read (and partially-downloaded). But honestly, don't take this the wrong way but I can personally remember what I've responded to, so it hasn't been an issue for me in particular. You can always set the flag to remind yourself - just tap and hold on the message and choose "set flag" or if you want a check mark, "complete flag".

Otherwise, there are 3rd party email clients that might offer those icons. I wonder if the pocket outlook in any of the 6.5 builds offer them?