Why am I prevented from sending any emails due to a pop-up message in upper left hand corner of screen?

This email problem is due to a pop-up message in upper left hand corner of screen that shows a pencil followed by the words "Write"=: (no subject) - Thunderbird" with a number of clickable and unclickable options listed below, none of which lead to any access to writing a new email? I booted my system twice and that made no difference. Also when I explored the options listed in the upper left corner of the screen, an old email address came up, which I no longer use more than 10 years ago but have never been able to delete from my system's memory.