If you need to check disk usage for a mail account, go to Mail tab and check the Usage gauge for the mail account in question. You can see actual usage for all mail accounts by clicking Refresh Usage Stats. Keep in mind that all mail sent to a mail account whose usage is greater than the mailbox size will be rejected. If this happens to you, simply upgrade your hosting plan to add more storage and email resources to your account.
To create a domain forwarder in Plesk: Start creating a new domain in Websites & Domains > Add Domain and specify the following: In the Domain name box, type the domain