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By default, the Relayer will attempt to deliver messages sent from its origin chain to any of the configured destination chains. Relayers may want to further filter the messages they attempt to deliver. For example, someone building an interchain app may want to run a Relayer that only delivers messages sent to that application. Similarly, some Relayers may wish to only relay messages to a subset of available chains. Relayers may optionally filter the messages they deliver by setting the --whitelist or --blacklist environment variables. See also the configuration reference’s whitelist section. These configs are stringified JSON objects with the following format: 
// A list of matching rules. A message matches if any of the list
// elements matches the message.
type MatchingList = Array<ListElement>;

// Matches a message if any of the provided values matches.
interface ListElement {
  originDomain?: NumericFilter;
  senderAddress?: HashFilter;
  destinationDomain?: NumericFilter;
  recipientAddress?: HashFilter;
}

type NumericFilter = Wildcard | U32 | Array<U32>;
type HashFilter = Wildcard | H256 | Array<H256>;

// 32-bit unsigned integer
type U32 = number | string;

// 256-bit hash (can also be less) encoded as hextype H256 = string;

// Matches anything
type Wildcard = "*";
Both the whitelist and blacklists have “any” semantics. In other words, the Relayer will deliver messages that match any of the whitelist filters, and ignore messages that match any of the blacklist filters. For example, the following config used as a whitelist will ensure the Relayer will relay any messages sent to Ethereum, any messages sent from address 0xca7f632e91B592178D83A70B404f398c0a51581F to either Celo or Avalanche, and any messages sent to address 0xca7f632e91B592178D83A70B404f398c0a51581F on Arbitrum or Optimism. 
[
  { "senderAddress": "*", "destinationDomain": ["1"], "recipientAddress": "*" },
  {
    "senderAddress": "0xca7f632e91B592178D83A70B404f398c0a51581F",
    "destinationDomain": ["42220", "43114"],
    "recipientAddress": "*"
  },
  {
    "senderAddress": "*",
    "destinationDomain": ["42161", "420"],
    "recipientAddress": "0xca7f632e91B592178D83A70B404f398c0a51581F"
  }
]
A valid config may look like 
--whitelist='[
    {"senderAddress":"*",
    "destinationDomain":["1"],
    "recipientAddress":"*"},
    {"senderAddress":"0xca7f632e91B592178D83A70B404f398c0a51581F",
    "destinationDomain":["42220","43114"],
    "recipientAddress":"*"},
    {"senderAddress":"*",
    "destinationDomain":["42161","420"],
    "recipientAddress":"0xca7f632e91B592178D83A70B404f398c0a51581F"}
    ]'
The blacklist supersedes the whitelist, i.e. if a message matches both the whitelist and the blacklist, it will not be delivered.