Microsoft has quietly dropped support for an older TLS version, Windows 7 pre-dates the TLS series so it needs to be installed as a windows update and I have had to add a registry key to get these systems working.
A lot of info here https://docs.microsoft.com/en-us/mem/configmgr/core/plan-design/security/enable-tls-1-2-client
Outlook doesn’t require .NET framework so ignore that in the article above
The patch is available from Microsoft here https://www.catalog.update.microsoft.com/search.aspx?q=kb3140245
The registry entry I am using is below:
Windows Registry Editor Version 5.00