case.cls

public class EmailToCaseParser {
    public static String parseEmails(String emailHeaders) {
    	Pattern p = Pattern.compile('Reply-To: \\s*([A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,})');
        Matcher pm = p.matcher(emailHeaders);
        string res = '';
        if (pm.find()) {
            res = pm.group(0).replaceAll('Reply-To: ','');
        }
        return res;
    }
}
        
trigger PopulateCaseWithReplyTo on EmailMessage (after insert) {
    for (EmailMessage emailMessage : Trigger.new) {
        if (String.isNotBlank(emailMessage.ParentId)){
            // EmailToCase : Case record created then EmailMessage record created
            String suppliedEmail = [SELECT SuppliedEmail FROM Case WHERE id = :emailMessage.ParentId LIMIT 1].SuppliedEmail; //Get the suppliedEmail From Case to edit it
            if (suppliedEmail.equalsIgnoreCase('[email protected]')){ //Only from Helpkit's email
                Case caseToUpdate = [SELECT Id, SuppliedEmail FROM Case WHERE id = :emailMessage.ParentId LIMIT 1]; //Get the case to update
                String email = EmailToCaseParser.parseEmails(emailMessage.headers); // Get the value for the key Reply-To
                caseToUpdate.SuppliedEmail = email;
                try {
                    update caseToUpdate; //Update the Case record which has been saved before the mail
                } catch(DmlException e) {
                    System.debug('An unexpected error has occurred: ' + e.getMessage());
                }
            }
    	}
    }
}