When posting a question you need to offer some information about your setup and situation. What OS are you using? What version of Thunderbird are you using? How many mail accounts do you have in Thunderbird ? Does this issue effect all of your mail accounts ? Do you get any error message when you try to Get Messages or send email ? If yes, please tell us what it says word for word OR better - post an image of that error message. Also post some information so we know what servers we are talking about. Servers sometime change requirements - this might be server settings or maybe they have changed Authentication methods or maybe the server now needs you use special passwords or switch on access. Hence the need to understand your basic setup and what Thunderbird is currently using. In Thunderbird * Help > Troubleshooting Information * Scroll down to 'Mail and News Accounts' section * Copy all the data in that section and paste info into this forum question. Also Check the following: Exit Thunderbird Access Firewall - this may be controlled by an Anti-Virus program. Thunderbird must be set up as an allowed program. If it's not set up as allowed then do this now.