Email from Mailgun's shared IPs is being flagged as spam by Hotmail and AOL, which may indicate insufficient warmup with those services (e.g. Hotmail is not familiar with some of their shared IPs yet, and suspects them of sending spam until proven otherwise).

Methodology: a mock verification email with properly structured markup, a verification button, and no marketing content, sent to each provider. https://www.inboxtrail.com/compare#how