How does the GFI MailEssentials greylist process emails addressed to multiple recipients?

Answer

GFI MailEssentials Greylist works by temporarily rejecting every email received which contains a unique triplet. The triplet consists of the sender SMTP server's IP address, the sender email address and the recipient email address. Once the triplet is received again after a number of minutes, GFI MailEssentials greylist will authorize the triplet, thus not rejecting it again.
When receiving an email with multiple recipients, GFI MailEssentials greylist will form a triplet for each of the recipients included in the email.

By default, GFI MailEssentials Greylist would allow an email through if at least one of the triplets is authorized. An advanced feature can set GFI MailEssentials greylist to only allow emails with multiple recipients if all triplets are authorized. This would mean that if 1 out of 10 recipients is part of an unknown triplet, the full email will be rejected, so the authorized triplets will not receive the email as well until it is retried.

Note: This feature can further increase the delay imposed by GFI MailEssentials Greylist

The advanced feature which sets GFI MailEssentials Greylist to require authorization of all triplets can be set as follows:

1. Open the Windows Registry Editor (Start > Run > type Regedit and press Enter)
2. Navigate to the following key:
   • x86 machines: [HKEY_LOCAL_MACHINE\SOFTWARE\GFI\GFI\MailEssentials\Config] 
   • x64 machines: [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\GFI\GFI\MailEssentials\Config]
3. Create or modify the DWORD value AcceptFullAuthorized
4. Change the value as follows:
   
   0 - Requires authorization of only 1 triplet
   1 - Requires authorization of all triplets
    
5. Re-start the GFI MailEssentials service to apply changes