For Plesk version 11 & 10
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1. Log into Plesk control panel.
2. Select the desired domain from "Domains" list.
3. Select "Change Settings" option from "Mails" tab.
4. Enable "Switch on greylisting spam protection for all mail accounts under this domain" option.
5. Click "OK" button.
For Plesk version 9
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1. Log into Plesk control panel.
2. Select the desired domain from "Domains" list.
3. Select "Mail Accounts" from "Mails" tab.
4. Select "Mail Settings" from "Mail Accounts" tab.
5. Enable "Switch on greylisting spam protection for all mail accounts under this domain" option.
6. Click "OK" button.