Anything else I can try?
Yes, far better to disable SPF checks on hostB when receiving mail from
hostA.
Thank you. Problem solved. For the benefit of others:
1) Add /etc/postfix/sender_checks file:
amazonses.com OK
2) Add check to smtpd_recipient_restrictions config in main.cf:
smtpd_recipient_restrictions = permit_mynetworks,
permit_sasl_authenticated, reject_unauth_destination,
check_sender_access hash:/etc/postfix/sender_checks check_policy_service
unix:private/policyd-spf
3) postmap sender_checks and reload postfix.
