From inside the app
Open Support → New Ticket. Pick a type (General Question, Suggestion, Issue, Bug, Security Incident, Data Export Request or Account Deletion Request), write a subject and message, and submit. Your ticket is visible to your organisation and to the DMARCS support team, who reply in the same thread.
Two things can also be requested directly from where you see them: an Expert Review of a sending source in Sending Sources, and a Takedown Request for a lookalike domain in Lookalike Monitoring.
Found a security issue?
Please don't email it to support. Use the Responsible Disclosure process on the Trust Center so it reaches the right people safely.
Is DMARCS down?
Check status.dmarcs.com before you call; if there's a known incident it will be there.