Sparrow behind a Proxy/Firewall
Sparrow might not work behind a Proxy/Firewall.
This might occur because Sparrow connects to the mail servers using IMAP and SMTP. This is the standard way to retrieve emails. Most of Proxy/Firewall will only allow web connections (HTTP).
You can ask your system administrator to allow the following outgoing TCP ports: 25, 110, 143, 465, 587, 993, 995.