My companies have 10 domains but we only use ONE of 10 domains for sending mail.
So in DMARC record for those 9 unused domains, should I just simply apply “p=reject” for them?
Unused domains should be marked as such using SPF and DMARC DNS TXT records:
@ IN TXT “v=spf1 -all”
_dmarc IN TXT “v=DMARC1; p=reject; rua=mailto:…”
The rua tag and reporting address can be omitted if you do not want to know about attempted abuse.
Something else that you might look at, if you haven’t already, is creating a null MX record for the domains
Thanks for your suggestion.
Unfortunately, my domain provider does not support null MX record.
I also read some articles which suggest to use private IP or “localhost.” instead of . in target host.