[donaldguy]

Very differently. Slack does threading on the basis of replies to a message and hides/collapses those under a thread, showing them only to people who expand and notifying only those who are already involved in the thread (by replying to it or being OP)

Zulip's threading is based, more like email, on a Subject which all messages (can) have. Though Zulip subjects are more likely to be one or two words.

In this way threading is tacked on to Slack whereas it is as at the heart of Zulip

[zAy0LfpBZLC8mAC]

Threading in email works via message-ids and references to preceding messages, nothing to do with subjects. That is, except as a workaround for garbage email clients and services too incompetent to use email correctly.

[donaldguy]

I mean, depends on your MUA in practice. I think by standard for server side threading— https://tools.ietf.org/html/rfc5256 — "base subject" is definitely a valid consideration. I only skimmed though. :-) feel free to extract a spec, I'm curious (but not enough to read 19 pages right now)