E-mail gives you access to a communication system that is fast, economical, and global. You can correspond with teachers who are working on the same topics or grades as yourself, and share strategies, frustrations, ideas, and insights. You can access experts in subject fields for information and advice. It's not just "conversation" either -- you can share text files, such as lesson plans, handouts, and bibliographies.
You can also link your students to other people through e-mail. For example, there are professionals who are willing to act as experts and will answer students' e-mail questions. Or, your students could join various lists, and pose questions, enter into discussions, and share information. Some lists are designed just for students, but they could join any list if the topic interested them.