VPNFilter
ASA does the auth part, authorization is passed over to ISE
Double certificate authentication
Certificate pinning
Duo security
Yubico two factor key
SPF, txt dns record to approve which IP's may send emails, https://mxtoolbox.com/
DKIM, asymetric key for autheticating the messeages https://community.sophos.com/kb/en-us/115300
DMARC, new standard for mail security, you will be informed when your domain will be misused.
pejaime.com descriptive text "v=smtp mx -all" pejaime.com descriptive text "v=spf1 a mx ip4:94.177.231.24 -all"