This answer explains how to configure the mail services for your VPS using the Plesk control panel

Your control panel works with Qmail mail server, which provides mail services for your domains. After installation the mail server is configured automatically. Although it will work immediately, we recommend you review the settings to make sure that they satisfy your needs:

1) Click the Server shortcut in the navigation panel.

2) Click the Mail icon in the Services group. The server-wide mail preferences screen will open on the Preferences tab.

3) If you wish to limit the size of an email message that can be sent through your server, type the desired value in kilobytes into the Maximum letter size box, otherwise, leave this field blank.

4) Select the mail relay mode.

With closed relay the mail server will accept only email addressed to the users who have mailboxes on this server. We don't recommend closing mail relay, because your customers will not be able to send any mail through your outgoing SMTP server.

With relay after authorisation, only your customers will be able to receive and send email through your mail server. We recommend that you leave the authorisation is required option selected, and specify allowed authentication methods:

• POP3 authorisation. With POP3 authorisation, once a user has successfully authenticated to the POP server, he or she is permitted to receive and send email through the mail server for the next 20 minutes (default value). You can adjust this interval by specifying another value in the lock time box.
• SMTP authorisation. With SMTP authorisation, your mail server requires authorization if the email message must be sent to an external address. Your mail server supports LOGIN, CRAM-MD5 and PLAIN methods of SMTP authorisation.

5) If you wish to protect your users from dictionary attacks by not allowing them to use simple passwords, select the Check the passwords for mailboxes in the dictionary option.

A dictionary attack is when someone tries to find a valid user name and password by running a program that tries different combinations of words in different languages. Dictionary attacks can be successful because many users choose their passwords carelessly.

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 part of their email address before the @ sign, or by specifying the full email address (e.g. username@yourdomain. com).

To avoid possible authorisation problems for email users who have different domains but identical usernames and passwords, we recommend you choose Only use of full mail account names is allowed.

Once you have set your mail server to support only full mail account names, you will not be able to switch back to supporting short account names until you make sure there are no clashes of usernames and passwords across different domains.

6) Click OK to submit any changes.