Ahh, ok. Then there is a misunderstanding. The SSL certificate you bought is most likely installed in a website? Then this is an pache ssl certificate. This has nothing to do with certificates for pop3 and imap.
See these instructions to create new ssl certs for courier:
If you need signed certs, you might have to let them sign at a ssl authority. But normally you will just import the ssl cert into your email client, so that you dont get a warning.