Know each other's work through publications, connect via email (ostensibly to ask for clarification on a detail, or to request a preprint), maybe meet at confer

Know each other's work through publications, connect via email (ostensibly to ask for clarification on a detail, or to request a preprint), maybe meet at conferences but not always. I've written a couple of papers with people who I've never met in person (and that's not uncommon).