Source: Plesk Help.
Selecting the Use of short and full names is allowed option will allow users to log in to their mail accounts by specifying only the left part of e-mail address before the @ sign (e.g. username), or by specifying the full e-mail address (e.g. username@your-domain.com).
To avoid possible authorization problems for e-mail users who reside in different domains but have identical user names and passwords, we recommend that you choose the Only use of full mail account names is allowed option.