Error :
The request failed.
The underlying connection was closed:
Could not establish trust relationship for the SSL/TLS secure channel.
The remote certificate is invalid according to the validation procedure.
Your On-Premises Exchange Server may need to import a valid web service certificate or a related signing or root CA certificate, so that ODMHE is able to verify the certificate of Exchange Web Service.
There are different procedures and requirements regarding creation and distribution of various types of certificates, depending on your environment.
Service Connection String specified in ODMHE may be associated with another service endpoint with a different certificate, hence update the connector with the correct URL that corresponds to the appropriate certificate.
If error occurs while trying to establish an SSL connection, then most likely server web certificate is self-signed or an incorrect host name is being used to connect, hence ensure that host name must match the name on certificate.
For example :
imap.example.com and example.com may point to the same server, but certificate is issued only to imap.example.com, so this should be the address to be used.
For a test outside of ODMHE, use this PowerShell command to verify on your HTTPS connection string :
Invoke-WebRequest -Uri <Service Connection String>
Quotes from ODMHE Security Doc :
1) ODMHE communicates with on-premise Microsoft Exchange servers over port 443 by default.
Port 80 may be used if you configures their on-premise server that way.
2) Self-signed certificates are allowed as well when connecting to on-premises servers.
Such certificate must be valid and its Issuer DN must match its Subject DN.
