Sent emails not appearing in sent folder in Outlook for Mac 2011

Hello!

I'm running Outlook for Mac 2011 version 14.4.5 on a MacBook Pro (Retina, 15 inch, early 2013).   The MacBook is running OS X Yosemite version 10.10.

Yesterday, my Outlook stopped placing sent mail in my sent folder.    

I have two different email addresses, one from a privately hosted server and the other is Optus (a major provider in Australia).   Both are POP accounts, and the same issue arises for emails sent from either server.

Here's more information:

• if I look at my inbox emails, it shows that the mail has been replied to,
• the mail is being sent and received (I know from talking to recipients)
• if I click on the 'show reply' button on a particular item of mail in the inbox, the sent email DOES display (so the emails are not being totally deleted from the computer, just not showing up in the Sent folder).

So that pretty much isolates the problem to some glitch in the software that is stopping items appearing in the sent mail folder.  

I did NOT change any settings for this to occur - it just started happening.

And it is NOT being caused by having a rule that sends all sent mail to the deleted folder.

Can anyone help with this?  It makes it impossible to work if you can't see what you've sent to people.

Many thanks,

Bruce