If you have a WHM installation, you may receive several emails (one for each service) from your WHM installation regarding expiring SSL certificates.
The subject of the email may state:
The SSL certificate for cpanel on xxx will expire in less than 30 days
And the message may state something like:
Your server’s SSL certificate for cpanel will expire in less than 30 days. You need to install a new certificate as soon as possible. You can install a new certificate using WHM’s “Manage Service SSL Certificates” interface: https://xxx:2087/scripts2/manageservicecrts (Main >> Service Configuration >> Manage Service SSL Certificates).
The services that use valid SSL or even self-signed certificates include the FTP server, IMAP server (Dovecot by default), SMTP server (Exim by default) and then the cPanel/WHM/Webmail interfaces themselves. So 4 emails in total.
If you are using an actual, valid SSL certificate, you will have to renew it through your SSL certificate provider in order to take care of this.
In most cases though, the SSL certificates are self-signed through cPanel/WHM themselves so you can go to WHM > Service Configuration > Manage SSL certificates and click “Reset Certificate” for each of the 4 services and cPanel will automatically generate a new, self-signed certificate for you.
All set and done!
Source: The SSL cert will expire in less than 30 days | cPanel Forums