I changed the calculation function to
and it worked?
What's the principle behind?
It's hard to say without knowing more about your app, but I would guess it has something to do with the List() function.
List ( RelatedTable::field )
produces a return separated list with one value from each related record where field is not empty. This list of values is not likely to be a valid format for emails as they normally would be separated by either commas or semi-colons instead of return characters.