smtpd_sender_restrictions = check_sender_access hash:/etc/postfix/sender_access, permit

mailfilter@iomart.com REJECT

postmap hash:sender_access

smtpd_recipient_restrictions = check_sender_access hash:/etc/postfix/sender_access

postfix/smtpd[2144]: fatal: parameter "smtpd_recipient_restrictions": specify at least one working instance of: check_relay_domains, reject_unauth_destination, reject, defer or defer_if_permit

smtpd_recipient_restrictions = check_sender_access hash:/etc/postfix/sender_access, permit

smtpd_recipient_restrictions = reject_unauth_destination, check_sender_access hash:/etc/postfix/sender_access

IMPORTANT: Either the smtpd_relay_restrictions or the smtpd_recipient_restrictions parameter must specify at least one of the following restrictions. Otherwise Postfix will refuse to receive mail:

    reject, reject_unauth_destination
    defer, defer_if_permit, defer_unauth_destination

smtpd_relay_restrictions = ...
    ...
    check_sender_access hash:/etc/postfix/sender_access
    ...