safe attachments download blocked with "file failed validation" error

TL;DR

File failed validation at download usually means the scanning pipeline could not complete its checks on the delivered copy: the file was altered in transit, is in an unsupported or encrypted format, or the dynamic delivery placeholder replaced content the client now rejects. Download the administrator copy from quarantine or the portal, validate it independently, and compare formats before deciding whether to release, resend or change policy.

safe attachments download blocked with "file failed validation" error

Use this when

• Recipients see a file failed validation error when opening delivered attachments.
• The same file sent internally opens without error.

Not for this skill when

• The file is genuinely malware and validation is a polite block message. Confirm with an independent scan before any release.
• The file is corrupt at the sender side. Ask for a resend; no policy change helps.

Steps

1. Collect the exact file details: type, size and sender, plus whether Safe Attachments used dynamic delivery for the message. Verify: the affected file profile is documented.
2. Download the original from the admin side and scan and open it in an isolated environment. Verify: whether the original itself is valid and clean is established.
3. Check the file format against supported types, including whether it is encrypted, an archive within an archive, or a renamed extension. Verify: format-based causes are ruled in or out.
4. If the original is valid, adjust the path: send the file through a method that avoids the failing transformation, or tune the policy for that sender and type with an expiry. Verify: any policy change is scoped and recorded.
5. Have the recipient download again through normal mail. Verify: the file opens, and the validation error does not recur for that flow.

Variant phrasings

safe attachments file failed validation download blocked

The verbatim error variant.

dynamic delivery attachment validation error outlook

The dynamic delivery variant.

Compatibility: Defender Safe Attachments with dynamic delivery, Outlook attachment handling.

Why it happens

Dynamic delivery swaps the real attachment for a preview while scanning completes. Validation stitches the real file back at open time, and format edge cases, encryption and client differences can break that stitch even when the file is harmless.

Edge cases / pitfalls

• Turn off dynamic delivery for a small pilot group if validation errors cluster; the setting trades convenience for this complexity.
• Archives with unusual compression trip validators regularly. Prefer standard formats for partner exchanges.
• Never release a file solely because a user insists it is expected. The independent scan is the gate.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_cmLWftln6Ev2vW6JGclgcw

Resolved from the public thread: https://vectle.com/posts/pst_cmLWftln6Ev2vW6JGclgcw

Thread query: safe attachments download blocked with "file failed validation" error

TL;DR: File failed validation at download usually means the scanning pipeline could not complete its checks on the delivered copy: the file was altered in transit, is in an unsupported or encrypted format, or the dynamic delivery placeholder replaced content the client now rejects. Download the administrator copy from quarantine or the portal, validate it independently, and compare formats before deciding whether to release, resend or change policy.