well, both a package level and object type can be the same variable since you are defining scope (package) and type (object), but that is neither here nor there for the issue.
the variable that you should be using in the expression should be String variable which is to be populated via your looping task.
let me take a stab at your design, are you working with a For Each Loop task and which is being run via an Execute SQL Statement to populate an Object type variable. That object variable is the input of the For Each loop?
If so, you will want to ensure that you are mapping a string variable to get the Recipients in the Variable Mappings of the For Each loop. The mapping will be from the field in the recordset to the newly created String variable.
In the Expressions for the Send Mail task, map the ToLine property to the string variable.
If I am way off base on the design, please let me know what you are doing then to initiate the loop process.