Signature request email could not be delivered

Thank you for your help.

I delegated a signature, but the person who I delegated did not receive an email. When I checked the history of Acrobat Sign, the signature was executed and there was a history that the signature email was sent to the person who delegated it, but the history says "Email sent to xxxxxxx@xxxxx.co.jp bounced and could not be delivered”, indicating that the item could not be delivered.

I asked my company's IT department to investigate, but there was no evidence that it had been blocked by a filter, and the mailboxes of the person who I delegated were not full. I don't know the cause and if you have any ideas, I would be very grateful if you could give me some advice.

One point of concern is that the member who delegated the signature is currently on a business trip to China. The following is included in the additional terms of use for Adobe Acrobat Sign.

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4.3 Regional Service Limitations. Unless use in a Restricted Country is specifically authorized by Adobe, you are not permitted to use Acrobat Sign in any Restricted Country. and any other country where access or usage is restricted by local laws.
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However, he is receiving emails via a VPN, so I believe I am able to receive them.