The icons on the right side can be also accessed via the mail menu and have the following functionality:

Fetch mail from server
If you click this icon it will fetch the mail from all available accounts. Click and hold for a popup menu with accounts to chose from.

Send queued mail
Sends all queued mail via SMTP via the mail server which was current when the mail was created

Create a new mail
Opens a new mail editor to start writing a new message

Abort
Cancel the currently running transfer

Remove mail
Removes the selected mail from the list