Mail merge with individual attachments
Word cannot attach a different file to each recipient, and never could. Why that limitation exists, and the three honest ways past it.
Guides
Outlook mail merge, explained properly
How to do the things Word's mail merge cannot, using the mailbox you already send from. Each guide covers the manual route first, so you can judge whether you need a tool at all.
Sending personalised invoices
From an accounting export to ninety invoices in ninety inboxes — from your own address, with a check before anything leaves.
Emailing payslips individually
The merge where a mistake is not an inconvenience. Matching by employee number, checking counts, and never sending a payroll email with nothing attached.
Mail merge without Word
The classic route needs Word, Excel and Outlook, and only works on Windows. What it takes to do it in one application — and what you give up.
Also worth reading: will merged mail reach the inbox? and setup & troubleshooting.