Check the following formula for Validation Rule to support 10 digit number or Email Address.

Validation Rule:

NOT ( 
OR ( REGEX( InfallibleTech__Employee_Name__c , '[0-9]{10}'), 
REGEX ( InfallibleTech__Employee_Name__c , '[a-zA-Z0-9._-]+@[a-zA-Z]+.[a-zA-Z]{2,4}') 
) )

Note:

Kindly replace your field name.