What you have is using the User object’s Id to compare against the Contact object’s Id.  That will always return an empty list.

You can select the ContactId from the User object and compare that as follows:

for (Case c : [
     Select Id, Contact.Name
     From Case
     Where 
         ContactId != null And
         ContactId In (Select ContactId
                       From User
                       Where Id = :UserInfo.getUserId())

]) {
    System.debug(c.Contact.Name);
}


Query the cases where case contact = logged in user's contact
I wanted to send a mail by telnetting through smtp server "telnet smtp_Server 25" and want to send mail. How is it possible..

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Thanks..
