There are various reasons why SAV fails. Here are a few:
The sender address does not exist as an e-mail account on the mail server registered for this domain. It may also simply be due to a misunderstanding or lack of knowledge. In such cases, you should notify the person responsible for technical administration of the sender’s domain/mail server.
The sender was intentionally falsified (SPAM).
Intentionally invalid addresses (e.g. firstname.lastname@example.org).
The sender’s mail server does not permit verification.