If you get the The server responded: 421 Cannot connect to SMTP server error when trying to send emails in Outlook, use these fixes.

Fix Cannot open Outlook window. The file .ost cannot be accessed, You must connect to Microsoft Exchange before you can use Outlook data file.

The inability to connect from a Vista PC to an Exchange Server may be due to a conflict with the Windows Firewall.

Microsoft says outdated Exchange servers cannot receive new emergency mitigation definitions because an Office Configuration Service certificate type is being deprecated.