I REALLY miss in Thunderbird the functionality of the former Mail Merge add-on, which unfortunately no longer exists: An email template with user variables that can access fields from an Excel spreadsheet. For example “Dear <FirstName> …” where <FirstName> is automatically replaced, row by row, with the value from the FirstName column in the Excel file. This way, you can create personalized emails and have them sent individually! It’s a shame that this is no longer possible — Outlook can do this!