[SalesForce] Is it possible to access the email of a Lead’s owner directly

Your problem is in this piece of code:

List<Contact> leads = [
    SELECT 
        Id, Email 
    FROM 
        Contact
    WHERE 
        Email IN :leadEmails
];

for(Lead singleLead:leads){
    singleLead.Status = 'Converted';
}

Notice how yourleads variable is a List<Contact>. You then try to loop over that list and cast each item in the List as a Lead. You can not do this. Those items are Contacts.

Try something like this out instead:

trigger ConvertLead on Lead (before insert) {
    List<String> leadEmails = new List<String>();
    for(Lead lead:Trigger.new){
        leadEmails.add(lead.Email);
    }

    List<Contact> contacts = [
        SELECT 
            Id, Email 
        FROM 
            Contact
        WHERE 
            Email IN :leadEmails
    ];

    Set<String> contactEmails = new Set<String>();
    for(Contact contact:contacts){
        contactEmails.add(contact.Email);
    }

    for(Lead lead:Trigger.new){
        if(contactEmails.contains(lead.Email){
            lead.Status = 'Converted';
        }
    }
}

First off, notice I changed the trigger to a before insert. The reason for this is because you will receive an error if you attempt to perform a DML operation on the object a trigger is run on in an after context. The reason this is not allowed is because it would put the code in an infinite loop.

Secondly, notice that I changed your variable names to be more descriptive. I renamed your Contact list variable to contacts. This is a core concept of Clean Code.

Thirdly, I used a Set to store all matching Contact emails in the database. This will only keep one value for each of the unique email addresses found by the SOQL query.

Finally, you will notice I removed all DML operations. With the trigger changing to before insert, there is no need to run a DML operation as the object will be inserted directly after the trigger runs.

NOTE: This is not production ready code. I have not attempted to run it and wrote it specifically on this site. You must write unit tests for it and do your own testing to verify it does what you intend. Good luck!

Please find the Email Template and the Trigger code. This sends an email when a record is created. Hope this will help.

Email Template - (Subjected - Record is created)

Dear {!CreatedBy} 
Your Response document is Created. You can download it from {!Document_Location_vgs__c}

Trigger Code

trigger SendEmail  on Response_Documents_vgs__c (after insert) {
    Response_Documents_vgs__c rdv= trigger.new[0]; 
    ID userid = rdv.CreatedById; 
    User activeUser = [SELECT Name,Email FROM User WHERE Id = :userid limit 1];
    String userName = activeUser.Name;
    String userEmail = activeUser.Email;
    String location = rdv.Document_Location_vgs__c;
    System.debug(rdv.Document_Location_vgs__c);
    System.debug(userEmail);
    EmailTemplate emailTemplate = [SELECT Id, Subject, HtmlValue, Body FROM EmailTemplate WHERE Id = '00Xi0000000Kbba']; //Email Template's ID
    String htmlBody = emailTemplate.HtmlValue;
    htmlBody = htmlBody.replace('{!CreatedBy}', userName );
    htmlBody = htmlBody.replace('{!Document_Location_vgs__c}', location );
    String plainBody = emailTemplate.Body;
    plainBody = plainBody.replace('{!CreatedBy}', userName );
    plainBody = plainBody.replace('{!Document_Location_vgs__c}', location );

    Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    mail.setSenderDisplayName('Salesforce Support'); //Sender's Display name
    mail.setReplyTo('support@salesforce.com'); //Sender's Email ID
    mail.setUseSignature(false);
    mail.setBccSender(false);
    mail.setSaveAsActivity(false);
    mail.setSubject(emailTemplate.Subject);
    mail.setHtmlBody(htmlBody);
    mail.setPlainTextBody(plainBody);
    List<String> sendTo = new List<String>();
    sendTo.add(userEmail);
    mail.setToAddresses(sendTo);       
    Messaging.sendEmail(new Messaging.SingleEmailmessage[] {mail});     
}

Email Received