A Local Mail Transfer Protocol (LMTP) developer is responsible for designing and implementing programs that facilitate the transmission of email messages across different servers. They work on the LMTP, which is a derivative of Simple Mail Transfer Protocol (SMTP), but with enhanced features. Their duties include creating and managing the mail server, debugging any issues, and ensuring the efficient delivery of emails. They also ensure that the protocol can handle large volumes of emails and is scalable. They work closely with other team members to integrate the LMTP with other systems and improve its performance and reliability.