While this information helped me with my Ionos email accounts, the directions will work for anyone who has to add extra SMTP Servers to send email from various email accounts. Email providers are requiring that the sending email match the receiving email to reduce the amount of spam being sent from their servers. So this is not a question--rather directions after 3 hours on the help line to figure out the proper procedure to add additional SMTP Servers. I wrote the direction below for my own use, but this may help someone else. The critical clue is to restart Thunderbird during the process so the new SMTP Server info gets 'remembered' (I am not a programmer, so my simple words). ===================== How to add additional SMTP Servers to Thunderbird (I am using v155.7.0) To meet Ionos’ new security settings, you must now have each of your email accounts use an outgoing server that matches the incoming server. 1. Select account (name of the specific email account/mailbox); click on Account Settings (far upper right hand corner 2. The Outgoing Server (SMTP) will have to match the incoming server, so you will need to add an Outgoing Server (SMTP) that identifies with your account. You cannot add an Outgoing Server (SMTP) from this window. 3. Scroll down left side column until you see Outgoing Server (SMTP). Select that item. That will open the window ‘Outgoing Servers (SMTP) Settings’ where you may add new SMTP Servers. 4. Select Add... This will open a window ‘SMTP Server’. Enter a unique description for the SMTP Server so you can easily identify it to match it to the respective email account. This is very important to you can later tie it to the specific email account. 4a. (This configuration is specific to Ionos as of Feb 2024) Add the server name (ex. smtp.ionos.com) and the respective Port (465), Connection Security ‘SSL/TSL’ and Authentication ‘Normal Password’. User Name is your specific email address that will match this SMTP Server. Save (OK). 4b. Restart Thunderbird. Yes, really, close and reopen Thunderbird. This action will force Thunderbird to save and then recognize this new SMTP Server. If you don’t restart Thunderbird, it will not find the correctly configured SMTP Server and you will go crazy. 5. Go to the specific email account that you have setup the Outgoing Server (SMTP) and again open Account Settings (far upper right hand corner. At the bottom of the Account Settings screen is the Outgoing Server (SMTP) and the default server. Click on the dropdown box (little arrowhead pointing down) of the SMTP server to get the newly added SMTP Server and select the SMTP Server that matches the email of this account. The information is automatically saved. 6. Go into your specific email account and send an email. Thunderbird will ask for the email password. Input the correct password and click on the box to save the password. Your email will now work using that specific SMTP Server you created unique to your specific email account. 7. Repeat for each unique email account. Remember to clearly identify the newly created SMTP Server name and to restart Thunderbird for each new Outgoing Server (SMTP) or Thunderbird will not have the settings to work correctly with that specific email account. This is to prevent spammers from sending tons of emails from various accounts.