As former email admin who has put up with this, you are breaking cardinal rule about mixing marketing and transactional emails.
Marketing should be different IP/Domain from transaction emails.
Marketing should be different IP/Domain from transaction emails.
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.
Yeah, I wish all marketing email would be clearly flagged as marketing. My bank does this and for them, marketing emails are actually more important than transaction emails, because marketing is creating value for them, and transaction notifications are costing them and only create value for me.
That is one of the ways how Microsoft acts evil. Email was never supposed to be this way. You have zero visibility into why some emails are blocked. One thing is to quarantine emails / put them into spam. But to reject them without any proper technical reason (like wrong SPF) is a nightmare from sysadmin standpoint.
This BS is pedaled by the email tools who want you to have less friction setting up DNS when onboarding to their tool (less conflicts on a new subdomain).
If you just check the latest version of Google Postmaster Tools, you can see sends from your root domain and subdomain cross-polluting your reputation in Gmail. They aren't dumb.
Have "mycompany.com" and "marketing-mycompany.com"?
How
Most of time, I would use mycompany.com for company stuff and mycompany.net for Application DNS and EMails.