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smtp.office365.com

Export StartTLS certificate from SMTP server

While configuring Office365 as the messaging (SMTP) server within Aruba ClearPass, I needed to upload the certificate from the StartTLS session to the certificate trust list from ClearPass. I had to export the certificate for smtp.office365.com via the following OpenSSL command:

openssl s_client -showcerts -starttls smtp -crlf -connect smtp.office365.com:587

After running the command, you will see some output like shown in the image.

openssl starttls

I copied the both parts between BEGIN CERTIFICATE and END CERTIFICATE to two different text editore files and saved them with the extension .cer. Next I was able to upload both certificates to the certificate trust list in ClearPass and configure the message server with StartTLS Connection Security