Microsoft Office Excel- The value you entered is not valid.

Hi Experts here. I have excel file which have columns Name, Address, Contact # and email add.
But I have a problem with email address. When I entered some other character I get this error.
How can I remove this message so that I can enter the character which is included in their email address?
Thanks.

Microsoft Office Excel
The value you entered is not valid.
A user has restricted values that can be entered into this cell.
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