Learned this the hard way. Sent maybe 50 outreach emails from the same domain handling transactional stuff (signup confirmations, notifications). Domain reputation tanked within a week and the transactional emails users actually needed started landing in spam.

The painful part is recovery. You can fix SPF/DKIM/DMARC in an hour but domain reputation takes weeks to rebuild. Microsoft's SNDS portal shows you almost nothing useful while you wait.

Set up the subdomain split from day one if you're a small SaaS. Costs nothing.