Looks to me as if Mail Merge could do this. At some point you have to declare some association between addressee and the appropriate file to attach. ''You'' may know what this is but conveying this to a dumb computer may be challenging. If you build up a CSV file to drive the merge then it seems you can declare the attachments there. Alternatively, I think you'd have to prepare a copy of each file to be attached, renamed in a way that would allow it to be selected against the name of the addressee. Maybe you can arrange attachment files in different folders and key the attachment to the folder names, if the mail merge attachments feature allows wildcards. https://addons.mozilla.org/en-US/thunderbird/addon/mail-merge/