This is using MS Exchange 2010 server with antispam. There is a mail blocked by antispam with the following messages:
The following message to <firstname.lastname@example.org> was undeliverable.
The reason for the problem:
5.3.0 - Other mail system problem 550-'5.7.1 Message rejected as spam by Content Filtering.'
Suppose the sender email domain is @sender.net, how to unblock it in antispam?