How can I get proof of sending an email and attachments for a legal matter

In Outlook and Microsoft 365, the closest to legal-grade proof of sending and content integrity is to use digitally signed (S/MIME) email. This does not retroactively prove past messages, but it is the supported way to strengthen evidence for future communications.

Key points for future messages:

1. Use a digital signature (S/MIME)
   • A digital signature verifies that:
     • The sender is who they claim to be (identity is validated by a certification authority).
     • The message content (including attachments) has not been tampered with after sending.
   • When a digital signature is inserted, Outlook automatically encrypts the email before sending.
   • Recipients can verify the digital signature on their side to confirm authenticity and integrity.
2. Obtain and install a digital ID (certificate)
   • A digital ID is required to sign messages.
   • Digital IDs are issued by certification authorities that independently verify the sender’s identity.
   • After installing the digital ID in Outlook, configure Outlook to sign outgoing messages that need legal evidentiary value.
3. Have recipients verify signed messages
   • In a dispute, the recipient (or a court-appointed expert) can:
     • Open the signed email in Outlook.
     • Verify the digital signature to confirm that the message and attachments are exactly as sent.
4. Subscription requirement
   • To use encryption with these features, the account must have a qualifying Microsoft 365 subscription.

For the specific past email already sent, Outlook itself cannot retroactively apply a digital signature. Evidence would then typically rely on:

• Message headers and server logs (from the mail provider) showing successful submission and delivery.
• Any read receipts or replies referencing the attached terms. These aspects are outside the provided documentation, but for future disputes, digitally signed email is the supported mechanism to provide strong technical proof of what was sent.

References:

• Learn about securing and protecting email messages in Outlook