There are various bugs in up-imapproxy which can crash it. Since up-imapproxy runs in a single process with each connection handled in a separate thread, any crash kills all the connections and stops listening for new ones.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/34967/up-imapproxy.txt
Source: https://packetstormsecurity.com/files/34967/up-imapproxy.txt.html