ksav not on sirtut
I have always paskened that by mezuzah (not tefillin), a ksav written under the sirtut (ie one or two mm under, or in between the two sirtututim ) is problematic. Recently someone told me that there are opinions that allow this, but I cannot find any. Can anyone shed light on the halacha lemaaseh re ksav not on sirtut by mezuzah?