I and several others seem to be experiencing this problem also.
It appears that any .xxx domain is not resolving in apples mail servers - and hence can't be sent email.
Sadly - the core problem here is that not all .xxx domain names constitute adult services, some people have them as a joke - and some (yes, rare - but true in our case) use them for business.
Blocking them from outbound mail seems to be redundant if it is aimed at stopping adult "spam" as the .xxx domains can send TO iCloud hosted email.
Apple Support are investigating - but it appears that they are blocking the XXX TLD in its entirety.