Hacker News Reader

I'm confused. In another comment you said "GMass doesn't actually send any emails through its own servers."

Let me clarify. GMass doesn't send any email, but we do have to transmit the email to someone that does send it, hence the bandwidth costs.

If it's client side, where does this communication happen?

It composes, personalizes, and "clicks" the send button. Google/gmail actually sends the email.