mail blocking on sme7 hasn't been updated.
and won't be for some time
And has limited usefulness in any case. Blocking of undeliverable mail is already a default feature in sme7.
the feature missing is
allow user@domain1 but block other domains
And I'd argue that that is of limited usefulness. Just don't tell people about other domains.
Sure, some spam might come, but it will come for user@domain1 anyway, and other spam detection features are already used to detect spam as much as possible.