This field controls the email address that the form shows as coming from. There is good reason not to use placeholders here, and, instead, to use an email address from your own domain. If you think about it, anyone could send an email on behalf of anyone else using your contact form if it arrives “From” the user’s inputted email address. This means it would be wide open to abuse. Also, most email servers now verify if the sender has the right to use that email address. And so, when the checks fail, the server identifies the mail as spam, based on a “forged sender address”. This hurts deliverability. The visitor’s name can still appear in the Email From Name field by using a placeholder, as seen below, but their email address should not be placed in the Sender Email field.
Best practice is to use something like email@example.com or firstname.lastname@example.org, which is the default sender address of WordPress transactional emails, such as password recovery and automatic update notifications. It should end with your domain name and extension, but what you put before the @ doesn’t matter. Do not use an existing address.