Sounds like you are offline. in the main mail window at the bottom is the online/offline indicator. The following images show this in Windows, the first little arrow thing turns on and off the spaces toolbar and the second changes the state of the application with regards to online. <img src=https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/images/2023-02-02-20-22-37-7f4616.png> appears when Thunderbird is online and connected to the internet and getting mail and capable of sending it immediately. <img src=https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/images/2023-02-02-20-22-48-27a213.png> appears when Thunderbird is offline and needs to connect to send or receive mail. In this start you get offered to "Send later" because there is no internet connection to send mail on. Clicking the indicator will toggle the state.