Form contents not forwarding to sender's GMail address

Hi
I have forms set up to redirect to PayPal.
After successful payment, the form submits to both recipient and sender - however, if sender uses a Gmail address in form, they do not get their copy of the form.

I have tested this both with existing forms (on client site) and a test form (on my site) with same results.

There are no conditions being used (to possibly filter or block certain email domains, etc) either.
I have also confirmed with our support that our server is working as expected with no issues sending to GMail accounts. For example, I can send a regula email to my Gmail account.

The issue seems to only arise when sending through the forms.

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