After researching this topic, I couldn't find a straight answer and tbh I'm a bit confused now.

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

My gut feeling would adapt they Facebook-way, but AirBnb's looks cleaner on the receiving end.

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