Mail merge documents corrupting: "Word found unreadable content" every time the merged file reopens, Word crashes opening the merge main document, or "Word was unable to open the data source"

What's actually wrong

Mail merge involves two files — the main document, which stores a live link to a data source, and the merged output — and either can go wrong. If Word crashes or errors when opening the main document, it's usually the data-source link (moved/renamed Excel file, .xlsx driver quirks, a locked shared workbook), not corruption at all. If the saved merged output throws "Word found unreadable content" on every reopen, a known Word bug has written invalid XML around the merge fields; the letter text is normally recoverable, but the field link often has to be rebuilt.

Free fixes — try these first

1. Detach the data source: open Word first, then the main document; when asked about running an SQL command, click No. Then go to Mailings → Start Mail Merge → Normal Word Document and save. The document opens cleanly from now on; re-attach the data file via Select Recipients when you next merge.
2. Fix the data file, not the document: if the crash happens when selecting or opening the data source, re-save the Excel source as .csv or .xls and re-attach it — .xlsx driver problems are a documented cause. Make sure nobody else has the workbook open.
3. For a merged file that errors on every reopen: let Word recover it (click Yes at the unreadable-content prompt), then select everything except the final paragraph mark, copy into a new blank document and save that — this workaround comes straight from users who solved it on Microsoft Q&A.
4. Open and Repair, then LibreOffice: File → Open → select the file → arrow beside Open → Open and Repair; if Word still refuses, open it in LibreOffice Writer and re-save as .docx.
5. Rebuild the template cleanly: paste the letter body (unformatted if needed) into a fresh document and re-insert the merge fields one by one — if the error returns after adding a particular field, you've found the corrupt one.

If the free fixes fail

When the merged file won't open at all — so there's nothing to copy out — or every recovered save re-corrupts on the next open, the field-code XML itself is broken; our analyzer repairs the document XML and strips the damaged field structures, showing an honest percent-recovered preview, with the caveat that merge-field links usually need re-creating afterwards.