Start from an export
An EML file usually contains one complete MIME message. MBOX stores several messages separated by envelope lines. MSG is an Outlook compound document and needs a different parser. Renaming a file extension does not convert one format into another.
Use bounded batches
Keep the source archive unchanged and inspect a small batch first. The browser limit is 64 MiB for a batch and 2,000 messages. Large PST or OST stores should be exported through a desktop client into smaller EML or MBOX batches.
Search operates over the decoded subject, sender, recipients and body. A filter changes the exported selection; it does not remove messages from the original file.
Start with a small representative batch
Choose Open my email files and select one ordinary message plus one message with an attachment. Run without a search filter first. Confirm the message count, subject, sender and attachment list, then inspect the plain-text preview. The built-in example contains a real MIME attachment and can help you distinguish a damaged source file from a general browser problem.
Once that works, select a larger EML batch or a supported MBOX export. Search for a distinctive sender address or a phrase from a known body. The search is case-insensitive and matches those decoded fields; it does not search attachment contents or perform OCR on attached documents. A zero-result export contains the index but no matching EML messages.
Treat parse failures as specific evidence
A missing header/body separator suggests an incomplete EML or the wrong format. A missing multipart closing boundary suggests truncation. Do not delete a source because the browser could not parse it. Re-export from the mail client, try one message at a time, or inspect the original locally. PST and OST are not supported here, even if a renamed extension resembles an accepted input.