I think it's an automated message triggered by biometrics... but it shouldn't be sent to applicants from the US because they submit their documents at a later date after biometrics.
In other countries, you apply online and then submit your application and documents in person at your biometrics appointment... so for those countries, the email makes sense - you attend biometrics, hand over the documents and then it triggers an email to say the documents have been received.
However, in the US, biometrics are taken by US immigration, before you submit your documents - so you attend biometrics and then mail the documents afterwards. Therefore, the 'package received' email should not be sent until the package has actually been delivered to Sheffield.
However, it sounds like the emails are being accidentally triggered when US applicants attend their biometrics instead.
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