Don’t think there is a pre-built option for just email besides the Zimbra modules and that really doesn’t fit here.
What mail software are you going to use for this? For example, you could use courier for imap service, postfix for smtp, and roundcube for the webmail. Within WHMCS, you would need a server module and on the server a script that the server module connects to to create, delete, etc the mailboxes. The server script would call courier commands to add users as well as any postfix commands for that side. Or where you planning on using something like google or other such mail services?