Hi,
It is likely that there is a required field that does not have a value.
It may be that the field is hidden, so that the user does not see it, nor can they input data. However even though the field is invisible/hidden, if it is set to required, then the submit by email will fail.
As you probably know, the required state is set in the Object > Value palette.
Also, it may depend on the version of LC Designer/Acrobat/Reader that you are using. At one stage the manner in which radio buttons were highlighted as being required was indistinguishable from not required. Because of this we sometimes use an asterisk in the caption to manually indicate required fields.
I'd suggest going through the form, looking for fields that are hidden and required.
Hope that helps,
Niall