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Outbound external messages are the messages that a contract can send to the outer world. They are primarily used for logging purposes. They can’t change any contract state or trigger a transaction. These messages are visible to anyone, and there is no guarantee that anyone outside the blockchain will process them. The structure is as follows: 
ext_out_msg_info$11
    src:MsgAddressInt
    dest:MsgAddressExt
    created_lt:uint64
    created_at:uint32
= CommonMsgInfo;

message$_
    {X:Type}
    info:CommonMsgInfo
    init:(Maybe (Either StateInit ^StateInit))
    body:(Either X ^X)
= Message X;
Unlike incoming external messages, a third party can’t manipulate the contents of outgoing external messages, so there is no need to sign them.
  • src — an internal address of the contract that sends the message. It has to be a valid address, but is not important, as it will be overwritten by the address of sender after the message is sent.
  • dest — any valid external address.
  • created_at — arbitrary value, overwritten with the created_at field of the transaction in which the contract sent this message.
  • created_lt — arbitrary value, overwritten with the created_lt field of the transaction, incremented by one.
When sending this kind of message, the contract may fill all of these fields with any values. src, created_lt, and created_at will be overwritten by the validator processing this message. For an outbound external message, init field also can store arbitrary data, as outbound external messages can’t initialize any contract.

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Message modes

As outbound external messages do not carry any Toncoin value, the price of their handling is paid similarly to SendPayFwdFeesSeparately mode. However, using any mode other than SendIgnoreErrors, SendBounceIfActionFail, or SendPayFwdFeesSeparately for outbound external messages results in exit code 34. In this case, regardless of whether the SendIgnoreErrors mode was used, the action phase will fail. Mode SendPayFwdFeesSeparately does not change the message behavior.

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Example in Tolk

In Tolk, outbound external messages may be composed using the createExternalLogMessage function. 
tolk 1.2

struct(0x10) ValueWasDeposited {
    value: int64
}

fun main(in: InMessage) {
    val msg = createExternalLogMessage({
        dest: createAddressNone(),
        body: ValueWasDeposited {
            value: 1000,
        },
    });
    msg.send(SEND_MODE_PAY_FEES_SEPARATELY);
}