Hi,
Welcome to Microsoft Q&A community.
It sounds like the issue is due to a mismatch between the cipher suites supported by the external supplier’s updated security settings and those supported by your customer’s Exchange 2019 server. Here are a few steps you can take to resolve this:
Verify Supported Cipher Suites on Exchange Server: Ensure that the Exchange 2019 server supports the same cipher suites that the external supplier has enabled. You can check and update the supported cipher suites in the registry at:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Cryptography\Configuration\Local\SSL\00010002
Enable TLS 1.2 on Exchange Server: Make sure that TLS 1.2 is enabled on the Exchange server. You can do this by modifying the registry:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Server]
"Enabled"=dword:00000001
Update .NET Framework and Enable Strong Cryptography: Ensure that the .NET Framework is updated and strong cryptography is enabled. You can enable strong cryptography by adding the following registry keys:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft.NETFramework\v4.0.30319]
"SchUseStrongCrypto"=dword:00000001
Check for Updates: Ensure that both the Exchange server and the external supplier’s systems are fully updated with the latest security patches and updates.
Review Event Logs: Continuously monitor the System Event Log for any additional errors or warnings that might provide more insight into the issue.
By aligning the cipher suites and ensuring that TLS 1.2 is properly configured and enabled, you should be able to resolve the connection issue.
If you need further assistance, feel free to post back!