After a move away from GFI MailEssentials and MailSecurity to ExchangeDefender, we implemented the settings as described in the IP restriction article on the Exchange Defender Blog. See the link to "Setting up IP restrictions for ExchangeDefender".
GFI MailArchiver was set to send mail to "localhost" and not the actual name of the server. We had to change this to the name of the server and we also had to add the IP of the server's internal network card to the allowed list of IPs to send to the server.
In my case, this was caused by GFI MailArchiver when the SMTP Host for sending notices and restoring emails was invalid. (Configuration/General/SMTP Settings).
Ensure that the anti-virus software is not scanning the GFI MailArchiver directory (<Program Files\Common files\GFI\>).