Split from this thread.

The error message telling you what's wrong will normally be shown on the Errors tab next to the Tasks tab. That's the one we need to see. However, if the tab is blank, let's try and generate one. You have cleared out the outbox, haven't you? You won't get anywhere if there are unsent messages in the outbox. You might have to disconnect from the Internet (e.g. switch off Wi-Fi or unplug the Ethernet cable) to be able to remove the unsent messages. Then - after reconnecting to the Internet if necessary - compose a new, simple test message with subject Test1 and send it to yourself. When you see the error message, click on it to select it, then use right-click to copy it and paste it into your reply here. We'll take it from there.