Did not have this problem for years, UNTIL Avast updated program and virus definitions.
Now using Outlook 2003, any attempt to send or receive via Gmail errors, as described by other users.
Disconnecting shields for either outlook.pst or
www.gmail.com does not fix the issue.
Disabling Avast shield DOES.
This proves that the issue is caused by the new Avast version.
Would like to know of any other solution before leaving Avast, thanks!