I know a bunch of sites who do that and the problem is usually that register emails get flagged by outlook and never arrive, causing a lot of support burden. Easier to then nudge people into the direction of Gmail or other providers that don't have these issues.

it was in the context of blocking email providers because of malicious mass signups (https://blog.pypi.org/posts/2024-06-16-prohibiting-msn-email...)