You're not doing anything wrong and Outlook appears to be quite picky about its certificates. Is this for imap or smtp. Do you which mail apps you are running? Probably the SMTP Server (postfix), then either IMAP and POP Server (cyrus-imapd), Kopano or Zarafa

If you are running the free packages you'll have postfix and cyrus-imapd. If so, go the the command line then:This will give copy your current certificate to *.old and give you a new self-signed certificate and key valid for 10 years. I then copy the certificate and key into a single file as that is how cyrus-imapd is set up at the moment. Then restart cyrus-imap either from the webconfig or from the command line with a "service cyrus-imapd restart"

If the Outlook is then complaining about your SMTP certificate, they are:
/etc/postfix/key.pem
/etc/postfix/cert.pem

Back these files up and you can then use the same .key and .crt files you created earlier by just copying them over and renaming them.

Note, in reality, for all of these you can use different certificate and key file names and just change the configuration files to point to the new certificates.

[edit]
I forgot to add, postfix and cyrus-imapd do not use certificates managed by certificate manager.
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