I have 4 email accounts in Thunderbird that I have used for more than a year. Suddenly last week I got the following error when sending from one of the accounts: "Sending of the message failed. An error occurred while sending mail. The mail server responded: Requested action not taken: mailbox unavailable. Sender address is not allowed..Please verify that your email address is correct in your account settings and try again." I checked this account's settings to make sure they were identical to those of the other three. Finally, I deleted the account and re-added using the same information. I was able to send email for about 12 hours. Then, the same error returned. What is wrong? What should I do? Note that I can send email from this account using webmail on ionos.com, my website host where all the email addresses reside. Here is the first part of the Applications Basic info for my system: Application Basics Name Thunderbird Version 115.7.0 Build ID 20240119095007 Distribution ID Update Channel release User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:115.0) Gecko/20100101 Thunderbird/115.7.0 OS Windows_NT 10.0 19045 OS Theme Launcher Process Enabled Multiprocess Windows 0/0 Fission Windows 0/0 Enabled by default Remote Processes 3 Enterprise Policies Inactive Google Location Service Key Missing Google Safebrowsing Key Missing Mozilla Location Service Key Missing Safe Mode false Memory Size (RAM) 6.9 GB Disk Space Available 714 GB