Hi Pan Fei,
What is the DLP version on which you are facing this issue ? Also share the OS details for Enforce and SMTP (Detection server ).
If it's 15.1 , We will need to follow below :
Stop all DLP Services on Enforce Box
On the 15.1 Enforce install, remove both mail-1.4.7.jar and javax.mail1.6.0 jar from the following directories:
<DLP_home>\Server Platform Common\15.1\Protect\lib\jar
<DLP_home>\Enforce Server\15.1\Protect\tomcat\lib
Place the mail.jar in the below location
<DLP_home>\Server Platform Common\15.1\Protect\lib\jar
<DLP_home>\Enforce Server\15.1\Protect\tomcat\lib
Start all the DLP Services on Enforce Box