If Thunderbird keeps timing out on a slow or high-latency connection even though the settings are right, the default response window is simply too short. Raising it in the Config Editor fixes it. Jump to your situation below or work through the methods in order.
When Thunderbird reports the connection to the server timed out on a connection that is otherwise working, the cause is often that the default response window is too short for a slow or high-latency network. Thunderbird waits a fixed number of seconds for the server to respond, and on a sluggish satellite, mobile or distant connection the server's reply can arrive just after that window closes, so Thunderbird gives up. The fix is to raise the mailnews.tcptimeout value in the Config Editor (Settings, General, scroll to Config Editor), for example from the default to a larger number of seconds, which gives the server more time to answer. This is distinct from error 10060, which is a firewall or antivirus blocking the connection outright; if raising the timeout does not help, treat it as a 10060 and check those.
When Thunderbird connects to a mail server, it waits a set amount of time for each response. That timeout is a sensible default for a normal connection, but it is fixed. On a slow link the round trip to the server and back can take longer than the default allows.
When the reply arrives a moment too late, Thunderbird has already concluded the server is not responding and reports a timeout, even though the connection is basically fine. Giving it a longer timeout closes that gap. This is different from a 10060 block, where the connection never gets through at all.
“This is the polite cousin of 10060. The connection works, it is just slow, and Thunderbird's patience runs out a second before the server answers. The fix barely anyone knows about is in the Config Editor: bump up mailnews.tcptimeout and suddenly Thunderbird waits long enough for that laggy satellite or mobile link to reply. If raising the timeout does nothing, then it is not slowness, it is a block, and I go chase the firewall and antivirus instead.”
mailnews.tcptimeout and increase the value, for example from the default to 300 seconds.If the connection works but is slow, the fix is more patience, not new settings: raise mailnews.tcptimeout in the Config Editor so Thunderbird waits long enough for a laggy server to reply. If a longer timeout makes no difference, the connection is being blocked rather than delayed, so treat it as error 10060 and check the firewall and antivirus.
If raising the timeout and ruling out a firewall do not help, the slowness may be on the server side or your wider network. Test mail on a different, faster network to confirm, and contact your provider if their server is consistently slow to respond. You can also submit your error to us for a tailored fix.