Have an infuriating problem with Outlook for Mac 2011. I have 4 IMAP e-mail accounts set up through Outlook. 

All 4 are with the same domain - provided by Yahoo Small Business.

One of the accounts is fine with very few problems. The others ask for password authentication constantly - sometimes to the point that you can't perform another action before the next error box pops up. 

Yahoo Support thought it might be a security issue and to also log in via the webmail so that the IP is recognised - but this doesn't help. 

What is the problem? It looks like Apple Mail runs the same set up fine. I don't want to switch but will have to soon as this is actually preventing me from getting work done.

It is driving me crazy.