To protect the reputation of your root-domain, using a subdomain with a different IP for transactional mails or newsletters seems to be the general advice (best practice).
A different IP makes sense, since a lot of blacklists and mailfilters are IP based.
But what about the following scenarios:
from: Cloudflare noreply@notify.cloudflare.com
Received: from mta-87-158.sparkpostmail.com
mailed-by: spmailtechno.com
Signed by: notify.cloudflare.com
from: discover@airbnb.com
Received: from o34.email.airbnb.com
mailed-by: email.airbnb.com
Signed by: email.airbnb.com
from: business-noreply@support.facebook.com
Received: from 66-220-144-144.mail-mail.facebook.com
mailed-by: support.facebook.com
Signed by: support.facebook.com
from: CloudPlatform-noreply@google.com
Received: from mail-sor-f69.google.com
mailed-by: scoutcamp.bounces.google.com
Signed by: google.com
from: support@sav.com
Received: from a9-42.smtp-out.amazonses.com
mailed-by: a9-42.smtp-out.amazonses.com
Signed by: sav.com
from: receipts@spaceship.com
Received: from o13.mailservice.namecheap.com
mailed-by: em4882.spaceship.com
Signed by: spaceship.com
The Glossary from PostMark says "Domain reputation is considered everywhere your domain is used...From,Return-Path,DKIM,links,headers,content..." [0]
Which (sub)domain does really count for the domain reputation of email senders?
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[0] https://postmarkapp.com/glossary/domain-reputation
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