I started getting this problem about 6 months ago. I have used Thunderbird on Linux for years with very few problems until about 6 months ago. What happens is that when I send a message I get a dialog box saying sending and it will hang there for a long time, sometimes permanently. The size of message doesnt matter. Sometimes I can click on cancel and then retry and I am sometimes told it was sent but not saved. Sometimes it did not get sent at all. It is getting more and more annoying. It seems to happen after long periods of inactivity. Its not DNS, I have changed settings and DNS servers make no difference. I even went through and deleted lots of email and still it persists. I am running no extensions.