Hi @jibin chacko ，
I noticed that your file format is PDF. Do you also receive this error if you send attachments in other formats, such as .xlsx files or .docx files?
If you are using O365, please see this article: invalid-message-ndr
This issue may occur if the following conditions are true:
- A transport rule is set up that has the Defer the message if rule processing doesn't complete option enabled.
- During text extraction, the scanning process timed out when inspecting the attachment.
It is recommended that senders try the following steps:
- Password-protect the message before you send it.
- Disable the Defer the message if rule processing doesn't complete option in the transport rule.
If the suggestions above don't work, please run a message trace, and provide full NDR reports (please be careful to hide your private information).
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