When you create, refresh, or clone a sandbox, user email addresses are modified in your sandbox so that users won’t accidentally receive emails while doing developments/implementation and performing testing in the Sandbox environment.
While doing development and testing, the existing Process Builders, Workflow Rules and Triggers may accidentally send emails to the users. In order to avoid this chaos, emails sent to email addresses ending with .invalid are ignored by the system.
Note:
1. User who refreshed the Sandbox email address won’t be appended with .invalid.
2. Email addresses configured in Custom Settings, Custom Metadata Types, etc. won’t be appended.
3. If you stuck with logging in, check this article – https://help.salesforce.com/articleView?id=000335702&type=1&mode=1
For additional information, please check the below reference link
