You can create a whitelist outbound for the domain under Compliance --> Whitelist --> Create. The 'Who' is 'To Domain', 'Data' is the domain name, 'Direction' is 'Outbound'. Under 'Queue' select 'Anti-Spam' and under 'Bypass' select 'DSN Bounce Verification Protection'.
For when they are sending you messages, you must do the whitelist by IP address. This is done the same as the outbound whitelist, except the 'Who' is 'IP Address' and the 'Direction' is 'Inbound'.
Remember to apply both rules.
Thanks for quick reply,
I already tried a whitelist entry, but choosed "Direction" is 'Both"...