What happens to messages that fail DMARC?

When a domain's policy is p=none, recipient services which analyze DMARC results will do nothing to the message, if DMARC failure is found.  That is not to say that there might be other causes for messages to not be delivered.  For example, a spam scanning engine might still decide that the message content is unacceptable.

Setting a p=quarantine policy should be assumed to result in all failing messages to be placed in quarantines at recipient hosts.  Usually this is a user-level spam folder, but sometimes this is a frequently-emptied system level storage only accessible by host admins.

Setting a p=reject policy should be assumed to result in all failing messages being rejected  by the recipient host. This is often more desirable than quarantine because the sender receives immediate feedback that the message was not delivered.  Quarantining is invisible to senders (the domain owner can see results a day or more later by use of DMARC reporting)