На самом деле если исправить терминологию, то получится так:

#Конфиг

smtpd_restriction_classes = service_a service_b service_c

smtpd_recipient_restrictions =
  ....
  check_recipient_access hash:/etc/postfix/check_domains
  ...

service_a = check_recipient_access mysql:/some/path/service_a.cfg.mysql
service_b = check_recipient_access hash:/some/path/service_b.users
service_c = check_recipient_access ldap:/some/path/service_c.cfg.ldap

#

/etc/postfix/check_domains:
    a.domain.tld service_a
    other.domain.tld service_a
    b.domain.tld service_b
    elsewhere.domain.tld service_b
    c.domain.tld service_c
    something.else.com service_c