Are you using Sendgrid or the default bubble apps.io sending domain? If so, the issue may be a Sendgrid issue not a Bubble issue. Sendgrid has terrible email delivery and terrible email reliability.
From what I’ve seen, if you aren’t on Sendgrid’s plan that lets you get a dedicated IP, most of their mail server IPs are blacklisted from Hotmail, Outlook servers.
In fact they didn’t have any account on the site… bad me! I added an alert message if user doesn’t have an account now (it is an internal tool for a company)