Staff from mailbox provider GMX/WEB.DE/mail.com (1&1 Mail & Media GmbH) have just announced on the Mailop list that they'll begin enforcement of DMARC checks in a phased rollout over the coming weeks.
What does this mean? It means that if you publish a DMARC policy of p=reject for your email domain, they will now reject email from that domain, during the SMTP transaction, if that mail does not pass email authentication checks and lacks DMARC alignment.
Messages that fail those email authentication checks will be rejected with an error stating "554 Transaction failed Reject due to domain's DMARC policy." Learn more about that here: https://postmaster.mail.com/en/case?c=r2001
If you're wondering how big this mailbox provider is, they say that they have 42 million active users.