> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Message expiration

> What message expiration is, why it exists, and how to reemit a previously sent message if it has expired and was never relayed.

# Message expiration

[Messages](/app-developers/guides/interoperability/message-passing) referenced between OP Stack chains have a limited validity period called the expiry window. Once this window elapses, the referenced message becomes invalid and can no longer be referenced.

For messages using [`L2ToL2CrossDomainMessenger`](/app-developers/guides/interoperability/message-passing), if a message expires before being referenced, developers can reemit the message on the source chain. This triggers a fresh `SentMessage` event, enabling the message to be relayed.

## The expiry window

The expiry window is an offchain constant that defines how long a cross-chain message or event emitted remains valid. For any chain in the [Superchain interop cluster](/interop/explainer#superchain-interop-cluster), messages must be referenced within 7 days (604,800 seconds) of the log being created.

After this period, a message can no longer be referenced unless the event is remitted.

## Reemitting an expired message

The `resendMessage` function on the [`L2ToL2CrossDomainMessenger`](/app-developers/guides/interoperability/message-passing) contract allows developers to reemit a message that was sent but not yet relayed.

This emits a new `SentMessage` log with the same content as the original message, enabling offchain relayers to pick it up again.

The process to reemit an expired message:

1. Call [`resendMessage`](https://github.com/ethereum-optimism/optimism/blob/a979a9444dbb482843f2a42f437ced54a8ac1053/packages/contracts-bedrock/interfaces/L2/IL2ToL2CrossDomainMessenger.sol#L110-L128) on the origin chain to reemit the message event. The contract verifies the message hash was originally sent. The call requires   [every parameter](https://github.com/ethereum-optimism/optimism/blob/a979a9444dbb482843f2a42f437ced54a8ac1053/packages/contracts-bedrock/interfaces/L2/IL2ToL2CrossDomainMessenger.sol#L110-L128) to rebuild the original message.

2. [Relay the new message](/interop/message-passing#executing-message) as normal.

Note: Re-emitting an already relayed message will have no effect on the destination chain, since the destination chain only honors the original initiating message.

## Next steps

* Learn how to [pass messages between blockchains](/app-developers/tutorials/interoperability/message-passing)