Mail data for domains on a cPanel account is stored under the account's home-directory data and is therefore included in the applicable home-directory/full-account backup data.
This is different from the separate partial backups for email forwarders and global email filters, which protect configuration rather than all stored messages.
Do not assume restoring a forwarder or filter backup restores mailbox messages. Likewise, restoring broad home-directory data can affect much more than one mailbox.
