package main

import (
    "fmt"
    "regexp"
)

func main() {
    re := regexp.MustCompile("\\s+")
    s := "MAIL FROM:<a@b.com> TEST"
    split := re.Split(s, 2) // number of splits, use -1 to split by all strings
    fmt.Println(split[0])
    fmt.Println(split[1])
}