how does relay_domains=$mydestination work?


out of curiosity: the compatibility readme documents "backwards-compatible
default setting relay_domains=$mydestination" and that the empty value
"default value has changed from "$mydestination" to the empty value. This

under which curcumstances can this cause an error?
if the domain is in "mydestination", it should be treated as local.

is it related to the parent_domain_matches_subdomains setting?