Adding Mailbox to existing forwarding address

1- Login to Plesk at: https://domain-ext.his.com:8443/
Replace with your real domain, substituting a - for the last . before the extension.

Using the Plesk Admin login provided when the account was created
if you forgot the password: use the "forgot password" link, to have a password link sent to the Plesk Admin email address.

2- After login in, select your domain and click on the Mail tab

3- Find your email account: user@domain.ext and click on it to open its settings.

Since the account already exists, you'll just have to check the Mailbox checkbox,

If you don't have the password for that account: enter a new one and make a note of it.

Click on the Forwarding tab: Remove the forwarding (unless you want to keep it).

4- Click OK to save the changes.

Now that you have the new mailbox on Plesk, webmail will be available at: https://webmail.domain.ext
Replace domain.ext with your own domain.

After that, you'll have to configure the new mailbox in your mail app.

Username = email address: user@domain.ext
password: The password you entered in step 3.
Server name: domain.ext.his.com
Incoming Settings: IMAP: port 993
Outgoing Settings: SMTP port 587 + Authentication (Password)
Username = email address: user@domain.ext
password: The password you entered in step 3.