Choose the “Reply All” option if others in the employer's organization were copied on the message.
Use the sender's name in the salutation (see below).
Thank them for the invitation to interview.
Include that you are “very interested in learning more about this opportunity.”
How do you reply to an email?
How do you respond to an email you received?
A simple reply stating "got it," "received it," or "thank you" might relieve my worries. So, yes, I do think it is polite and appropriate to acknowledge receipt of valid emails as soon as possible.
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