There is disclosed a source messaging system having a queue for receiving messages. The source messaging system has means for determining whether a message should be permitted to become INDOUBT. This is done by retrieving a value denoting the maximum number of messages that may be permitted to become INDOUBT at any one time; determining whether the message falls within the range denoted by the value; and responsive to determining that the message falls within the range, permitting the message to become INDOUBT.