Occasionally I'll post something here for the sole purpose of giving myself a point of reference for future configurations, or to simply fluff my post count since they're so infrequent. That being said...

To configure email alerts for AnyConnect client activity on the ASA platform...

Enable logging with timestamps.

logging enable
logging timestamp

By default, syslog messages will mask the username in the event of a failed authentication attempt. I prefer to log the username of the failed authentication.

no logging hide username

Create a list of syslog messages that should trigger email alerts.

logging list VPN-EVENTS message 113005
logging list VPN-EVENTS message 113019
logging list VPN-EVENTS message 113039
Message ID Description
113005 AAA authentication on a connection has failed.
113019 AnyConnect session ended.
113039 AnyConnect session started.

Define the sender and recipient addresses, referencing the configured logging list VPN-EVENTS.

Note: Subtitute the addresses with those that are valid for the environment.

logging mail VPN-EVENTS
logging from-address anyconnect@example.com
logging recipient-address helpdesk@example.com level informational

Define the email server.

smtp-server DMZ 172.31.222.11

An email will be sent to the defined recipient address whenever an AnyConnect VPN client connects, disconnects, or fails authentication.

Expand for full config example
logging enable
logging timestamp
no logging hide username

logging list VPN-EVENTS message 113005
logging list VPN-EVENTS message 113019
logging list VPN-EVENTS message 113039

logging mail VPN-EVENTS
logging from-address anyconnect@example.com
logging recipient-address helpdesk@example.com level informational

smtp-server DMZ 172.31.222.11