Please repeat the
"Mark as read" button link at end of each list.
You usually decide to mark a series of messages as read once you've read it,
and you usually are at the end of a list once you've read it.
Having to scoll back for marking them read is a waste of time.
I also suggest to convert this link to a real link.
Making it a pseudo button or button lookalike was a bad design decision, imho.
If it was a link, one could use the "open in another window/tab" feature of
browsers having that, and continue reading with other messages unmolested
from pages with newly arranged lists of message popping up, while
otherwise one has to wait for them to display which is another
real waste of time.