SharePoint 2016 send email error, certificate validation errors

Question

I'm facing an issue when trying to send emails from SharePoint, I get one of the error messages below:

An operation failed because the following certificate has validation errors:
  Subject Name: CN=mailservername
  Issuer Name: CN=mailservername
  Thumbprint: 645B2D4B72573478DBD162F39AAE0260B25F6582
  Errors: SSL policy errors have been encountered.  Error code '0x6'..

An operation failed because the following certificate has validation errors:
  Subject Name: CN=mailservername
  Issuer Name: CN=mailservername
  Thumbprint: 645B2D4B72573478DBD162F39AAE0260B25F6582
  Errors: The root of the certificate chain is not a trusted root authority..

I have a single server farm, with SharePoint server 2016

When I configured the outgoing emails with the exchange server as provided by the IT team (mail.organizationname.gov.xx) or the server IP, I get the first error message.

When I configured it with the mail server name as mentioned in the error (either as is, or with domain (FQDM)) I get the second error !

Any help would be highly appreciated

Thank you

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Edit: I'm sending the emails via PowerShell

Answer 1

1. You need to set SSL to Yes in Outgoing emails and Create TLS communication between SP and Exchange servers. Manual here. They're resolving the same issue with certificate in 2nd half of manual.

Hope it helps!

Answer 2

Thanks to Zdeněk's answer, after referring to the article above, it turns out since I'm contacting the exchange server with TLS, the SendEmail via PowerShell won't work, and sending an email from SharePoint itself was already working, and I only tried PowerShell for testing purposes (Which I forgot to mention it in my question)

Thanks alot Zdeněk for your replies