This is one of two things. Try the easy fix first:
Close the mailfile (not Notes, just the mailfile)
Go to the workspace
Remove the mailfile icon
Compact the workspace (don’t skip this; it’s key)
Delete bookmark.nsf, cache.ndk and (if it won’t totally mess up the user), desktop6.ndk
Readd and reopen the mailfile
If this doesn’t work, then the view corruption is in the mailfile itself.
From the Administrator Client, run fixup, compact, updall
Often I’ve found that fixup alone won’t do the job; you need the whole sequence.
Hope this helps.