Bug 2278

When an operation that downloads messages from a POP3 server is in progress, a concurrent POP3 connection can be opened that will download the same messages, resulting in duplicates. When a manual POP3 download is started from the UI, e.g., using the "Get All" button, the button is greyed out while the operation is in progress, which is the desired behavior. However, a concurrent download from a POP3 server occurs when an automatic mail check is in the process of downloading messages and the user presses the button, or, when an automatic mail check takes so long, that another automatic mail check starts before the previous one has finished. Here, the desired behavior would be to prevent opening another session to the same POP3 server while a session exists.