Please see
http://www.codecrafters.com/AbilityMail ... utorialSSL and
http://www.codecrafters.com/AbilityMail ... rtificates for information on SSL, creating certificates and importing them into Ability Mail Server.
You can use Ability Mail Server to create a self-signed certificate which will also create a Certificate Signing Request (.csr) file. You can either use this with GoDaddy or they probably have the option just to fill in the details directly on their site to create the certificate directly. Make sure the common name of the certificate you create matches exactly the domain you will use for your email server MX record (e.g. mail.codecrafters.com) otherwise the certificate will not work. The rest can be anything you want really.
The important thing is that you purchase an RSA base64 encoded certificate (X509) as this is the right kind of certificate for use with Ability Mail Server.
You can either import separate private key (.key) and certificate (.crt or .cer) files or use a combined .pem file that contains both private key and certificate.