Try this if you are getting this error with Microsoft Outlook or Outlook Express.

Getting ‘error 0x800CCCOF’. the connection to the server was interrupted?

1. If you are using Firewalls or Norton internet security etc, please disable them and try sending emails.

2. Follow these instructions:
1. At Tools menu, choose Account.
2. Choose Mail.
3. Click double to your Account.
4. Choose Servers.
5. Click My Server Requires Authentication.
6. Choose OK to complete.
Then try sending email.

3. Check this document for more reasons and solutions:
https://support.microsoft.com/?kbid=813514