Office for Mac 2011 – Outlook error message: “Connection to the Server failed or was dropped”

Hi, I have been getting this error message in Outlook ("Connection to the Server failed or was dropped") when I attempt to send emails. I am receiving some emails, but not all. I am not getting test emails that I send to myself.

 When I send an email to certain recipients, they don't go through. They don't show up in my sent folder either. I have checked my account settings, but am not sure what to change.

I'd appreciate any help you can offer!

Thanks,
Katia