I will make a few assumptions here as you have not defined what you have and do not have available in your question.
Things you need to do / you have already:
- Define XML Format
- Create class that XML can be serialized to (You can use XSD.EXE or do it manually)
- Split XML containing email addresses into manageable chunks (Memory management concerns)
- Load one chunk at a time
- Use code to iterate through the list of email address in chunk and send email
Things you need for application to work:
- Email file loader (Includes deserialization engine)
- Email sending engine
Things you need to set up data:
- Utility to read in text file and output in XML format (Includes serialization engine).