[pornel]

It's not blocked. It's just that some very badly written proxies can try to buffer the "whole" response, and SSE is technically a never-ending file.

It's possible to detect that, and fall back to long polling. Send an event immediately after opening a new connection, and see if it arrives at the client within a short timeout. If it doesn't, make your server close the connection after every message sent (connection close will make AV let the response through). The client will reconnect automatically.

Or run:

    while(true) alert("antivirus software is worse than malware")