re : I created the merged emails with the Send Later option, figuring I could send them whenever I chose. The merged emails landed in my Outbox.

That is correct because all of the emails have been processed with send data, but are just being delayed in the Outbox. But you have control over Outbox.

re :But then TB began to automatically send out the emails

Sounds like you started up Thunderbird and it started to act on a preference setting to auto send emails in Outbox. Check this setting:

• Menu app icon > Preferences > General
• scroll to 'Network & Disk Space'
• 'Offline' section
• Click on 'Offline..' button

'Send unsent messages when going online?' What is selected ? Is it 'Yes' - this would explain why it started sending automatically whenever you either moved from offline to online or when you started up thunderbird in online state (online is the usual startup mode)? Suggest you select 'No' or 'Ask me' then click on 'OK'

re: don't know how to get the merged emails to land in a folder that won't get sent out immediately.

Mail Merge offers 3 options: 'Send Now' - do not advise this. 'Send Later' - puts prepared to send emails in 'Outbox'. But when you choose to send unsent messages, all of them will get sent. If you have more emails in Outbox than what your smtp server will allow in a given period, you could end up unable to send as limits were reached. 'Save as Draft' - this will put all emails into the account 'Drafts' folder. This means you will need to select the 'Drafts' folder and double click on each email to open in a Write window and then click on 'Send' or 'Send Later'. It is more tedious, but it will allow you to send some immediately and also store some ready to send in Outbox which would go as a batch - it allows you to control how many emails are in that batch, so you do not exceed smtp server limits. All others can remain as a draft until you have emptied the Outbox, at which point you can add some more using the 'Send Later'.

Thanks for your help. I am also seeing a mix-up when it comes to received emails. In the top window I see the send and subject line, but the bottom windows shows the body of an email from someone else (please see image)

I wonder if TB is corrupt somehow?

I notice you are using the Quick Filter. Stop the filter.

See if index is corrupt. right click on Inbox and select 'Properties' Click on 'Repair Folder' button click on 'OK'

select another folder and then reselect the Inbox.