External referenced DTD in XML

test.xml:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE email SYSTEM "test.dtd">
<email>
<von>[email protected]</von>
<zu>[email protected]</zu>
<titel>Hello</titel>
<text>Dear John....;-).</text>
<prior type="schnell"/>
</email>

test.dtd:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE email [
<!ELEMENT email (von,zu,titel,text,prior)>
<!ELEMENT von (#PCDATA)>
<!ELEMENT zu (#PCDATA)>
<!ELEMENT titel (#PCDATA)>
<!ELEMENT text (#PCDATA)>
<!ATTLIST prior type CDATA #REQUIRED >
]>

Error Code in test.dtd

The markup declarations contained or pointed to by the document type declaration must be well-formed. [2]

Please help!!

You have duplicate DOCTYPE declarations. If you want to reference an external DTD:

test.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE email SYSTEM "test.dtd">
<email>
<von>[email protected]</von>
<zu>[email protected]</zu>
<titel>Hello</titel>
<text>Dear John....;-).</text>
<prior type="schnell"/>
</email>

test.dtd

<!ELEMENT email (von,zu,titel,text,prior)>
<!ELEMENT von (#PCDATA)>
<!ELEMENT zu (#PCDATA)>
<!ELEMENT titel (#PCDATA)>
<!ELEMENT text (#PCDATA)>
<!ELEMENT prior EMPTY>
<!ATTLIST prior type CDATA #REQUIRED >

If you want your DTD as part of the XML file (internal subset):

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE email [
<!ELEMENT email (von,zu,titel,text,prior)>
<!ELEMENT von (#PCDATA)>
<!ELEMENT zu (#PCDATA)>
<!ELEMENT titel (#PCDATA)>
<!ELEMENT text (#PCDATA)>
<!ELEMENT prior EMPTY>
<!ATTLIST prior type CDATA #REQUIRED >
]>
<email>
<von>[email protected]</von>
<zu>[email protected]</zu>
<titel>Hello</titel>
<text>Dear John....;-).</text>
<prior type="schnell"/>
</email>

NOTE: You're also missing an ELEMENT declaration for your prior element.