After being rejected, the message is routed back to the sender while also informing them that Domino didn’t accept the email.  However, people can request that you add them to the list of “safe” senders, allowing you to receive their future emails.

To activate this setting, you need to modify the appropriate setting in the Configuration Settings document.  Navigate to the Router/SMTP -> Select Restrictions and Controls -> SMTP Inbound Controls tab.  You will find six additional sections that you can use to configure the SMTP protocol.

More information is available here.