I previously offered the following suggestion (which had worked
for me) to another poster with the same problem:

"If you are using Word as your email editor in Outlook, try
turning off that feature, Restore Down your Word window, Maximize
your Word window, and then turn on Word as your email editor in
Outlook again."

and he subsequently replied:

"Worked great for me.  but i had to turn off Word as Editior,
close outlook, close word, open word, maximize, open outlook,
turn on feature.  The only difference was shutting down outlook
for a second.  Everything now works fine."

So now you have two possible solutions, both of which have fixed
the problem for different people.