I have a few users that run Office for Mac and occasionally they will get the following message.  

"An operation on the server timed out. The server may be down, overloaded, or there may be too much net traffic"

When they get the error message Outlook will quit working for them and they have to wait to reconnect.   The issue just started a few weeks ago and is only happening to a few of our users.

Hi,

Do you also see any Error Code about this issue?

Based on the error message, please try the steps below:

Step 1: Verify that you can connect on the webFirst, test to see you can sign in using the web based application.  For example, if its a Gmail account, visit gmail.com and Sign in. If you were able to sign in then proceed to the next step.  If you are not able to sign in, contact your email client to obtain correct user name and password.

Step 2: Change the SMPT port to 587
Change the Outgoing Server port to 587, check the result.

Regards,

Melon Chen
TechNet Community Su