Mandatory certificate verification is historically not viable for Internet mail delivery without additional information, because many certificates are not verifiable and few want email delivery to fail in that case. As a result, most email that is delivered over TLS uses only opportunistic encryption. DANE is a proposed standard that makes an incremental transition to verified encryption for Internet mail delivery possible. The STARTTLS Everywhere project uses an alternative approach: they support a “preload list” of email servers that have promised to support STARTTLS, which can help detect and prevent downgrade attacks.