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.
[/edit]

This is great. Thanks for the information. At least I know I'm not crazy.

I am using IMAP and you are correct in assuming that both it and SMTP are the free versions. I am actually getting two certification warnings back to back as soon as Outlook opens so, maybe one is for IMAP and the other is for SMTP? This is just a guess based on the fact that, if I answer No to both warnings, I can not send or receive emails and if I answer Yes, sending and receiving work fine.

I will try your solution and post back with the results.

Cannot edit or create certificates

I am using ClearOS Community release 6.9.0 (Final) and I am getting a certificate warning when I login to Outlook. I states that my localhost.localdomain certificate is out of date, witch it is. Outlook works fine but, I have answer yes to two popup warnings about the certificate not being able to be verified.

When I open the Clear OS console and choose Certificate Manager under System, I do not see an option to edit the certificates that are already there nor do see an option to create a new certificate. I know a little bit more than nothing about certificates so I pretty sure I'm doing something wrong.

Any help on how to get the options to edit or create certificates would be appreciated.