Once you have created a mailbox in Plesk e.g. email@example.com, you can create email aliases in Plesk for the mailbox e.g. firstname.lastname@example.org so that any mail sent to one of the email aliases will be delivered to the mailbox.
Please follow the link below for a tutorial:
How to create an email alias in Plesk
For further support and assistance on setting up your email review the following tutorials:
If you have any problems with setting up your email services please let us know.