The SendAs audit event is logged when someone uses the send as permission to send a message from an Exchange Online mailbox. The events are stored in the Office 365 audit log and can be found there with an audit log search. However, things aren’t as straightforward as they are on-premises because some other types of delegated messages turn up in searches. Fortunately, we have a script to help.
https://office365itpros.com/2020/04/09/sendas-audit-exchange-online-mailboxes/
