您在错误的地方使用了 async await。包含异步函数的函数必须使用 async 关键字

await 关键字需要用于返回 Promise 的表达式，尽管 setState 是 async ，但它不返回 Promise，因此 await 不适用于它

您的解决方案将类似于

componentDidMount() {
  let mails = [];
  axios.get('/api/employee/fulano')
    .then(res => this.setState({
      employees: res.data
    }, async () => {

      const mails = await Promise.all(this.state.employees.map(async (i) => { // map function contains async code
        try {
             const res = await axios.get(`/api/status/${i.mail}`)
             return res.data;
        } catch(err) { 
            console.log(err)
        }
      })
      this.setState({ mails })
    }))
    .catch(err => console.log(err))
}

将 async/await 与 .then/.catch 混合使用并不是一个好习惯。请改用其中一种。下面是一个示例，说明如何使用 仅 async/await 和 仅 一个 this.setState() 来实现此目的（参考 Promise.each 函数）：

componentDidMount = async () => {
  try {    
    const { data: employees } = await axios.get('/api/employee/fulano'); // get employees data from API and set res.data to "employees" (es6 destructing + alias)

    const mails = []; // initialize variable mails as an empty array

    await Promise.each(employees, async ({ mail }) => { // Promise.each is an asynchronous Promise loop function offered by a third party package called "bluebird"
      try {
       const { data } = await axios.get(`/api/status/${mail}`) // fetch mail status data
       mails.push(data); // push found data into mails array, then loop back until all mail has been iterated over
      } catch (err) { console.error(err); }
    })

    // optional: add a check to see if mails are present and not empty, otherwise throw an error.

    this.setState({ employees, mails }); // set employees and mails to state
  } catch (err) { console.error(err); }

}

这应该可行：

    componentDidMount() {
        axios.get('/api/employee/fulano')
         .then(res => this.setState({
          employees: res.data
         }, () => {

           this.state.employees.map(i => {
           axios.get(`/api/status/${i.mail}`)
             .then( async (res) => { // Fix occurred here
                let mails = [].concat(res.data)
                await this.setState({
                  mails: mails
             })
          })
          .catch(err => console.log(err))
        })
      }))
         .catch(err => console.log(err))
     }