When a sandbox is refreshed, salesforce add ".invalid" to all salesforce users, so we will need to remove this and ensure that users can log into the sandbox. We need to do this for all users that need access to the sandbox.

Because we use the "Integration" user for all our external processes, the same should always have access to our QA and UAT sandboxes, so we will show how to enable a user using integration as an example.

Go to the user page and remove the ".invalid" text at the end of the email address.
This is going to send an email to the selected address asking to confirm it, after that the user should be able to log into the sandbox with the same password as Production.
In case the user is unable to log in, we can just reset the user password clicking on the "Reset Password" button.

We have a lot of processes that send emails to the "preffered email address" field for contacts on the system, so it is recommended to update all contacts. Just add ".invalid" at the end of the personal and work email addresses using dataloader.

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