• Login to: http://www.example.com:8443/
• Set to Unlimited Resource Usage:
  • Domains > www.example.com > Statistics: Resource Usage > Expiration Date = Never Expires

Source: Plesk Help.

• Server > Mail
• Select the mail account name format.

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.

• Domain > Mail.
• Reject.