To receive new messages from the mail server and to send messages you have written that are in the local Outbox, click the Send / Receive button in the toolbar, or press F9, or choose FileSend / Receive from the main menu.

To send and receive only for one of your mail accounts, or to only send or only receive, click the small down-arrow next to the Send / Receive button and choose the corresponding option.

If you just want to send messages which are in your Outbox, you can right-click on the Outbox and click Flush Outbox.