Mail sporadic on wifi with range extender

I have a wifi range extender and an MacBook Air running OS X 10.9. I notice that in the room above the range extender, while my wifi signal (as shown by the little wifi icon in the menubar) is strong - all bars full, the Mail application is acting weird - some emails don't download the body of the email, some don't get sent, the Inbox doesn't update promptly, etc. Outlook 2011 mail does the same thing. If I open up a Firefox browser, I can browse, although not super-fast, so I know the network connection is there - it's just for some reason the mail clients aren't liking it. When I go downstairs, everything proceeds properly. How do I debug this - surely there's a way to make the mail clients work properly if there's enough wifi for a browser to work? Are there settings on the wifi extender, or on the laptop, to make the email act more robust?