On February 20 I was able to send email. An update was applied to Thunderbird that week. On February 23, after the update, I was unable to send email. I have been unable to send email ever since. My incoming email works fine. The error message I receive is: ----- Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) smtp.q.com failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again. ----- I contacted CenturyLink and we tested the SMTP server. They determined that it is working properly. Since the settings were correct on Tuesday, they were correct on Friday. I checked them anyway, and there is no problem: smtp.q.com, port 465, SSL/TLS, Normal password. The online help suggested that I delete my saved SMTP password. This had no effect either. I tried the TLS change from an older Thunderbird update. That also had no effect. I am using Windows 10 Pro version 22H2. Windows Defender Firewall is on. I am unable to find a version number for that. I am using the Windows Antivirus with the 3/10/2024 8:47 AM update applied. I am using Thunderbird 115.8.1 (32-bit). My ISP and email provider is CenturyLink. At this point my outgoing email is unusable. Hopefully someone there will have an idea what I can try next.