DMARC (Domain-based Message Authentication, Reporting, and Conformance) is a security protocol that helps prevent spam and phishing attacks by verifying the authenticity of emails. A custom Return-Path plays a crucial role in DMARC alignment, as it ensures that the domain in the Return-Path matches the domain in the "From" address, allowing DMARC to verify the legitimacy of the email and pass or reject it accordingly. When an ESP (Email Service Provider) like Postmark is used to power email for a domain, setting up a custom Return-Path helps align with SPF records, making it easier to implement DMARC policies. While not all ESPs support custom Return-Paths, DKIM records can still be used to achieve alignment and prevent spam messages from reaching recipients' inboxes.