address         =   mailbox / group
   mailbox         =   name-addr / addr-spec
   name-addr       =   [display-name] angle-addr
   angle-addr      =   [CFWS] "<" addr-spec ">" [CFWS] / obs-angle-addr
   group           =   display-name ":" [group-list] ";" [CFWS]
   display-name    =   phrase
   mailbox-list    =   (mailbox *("," mailbox)) / obs-mbox-list
   address-list    =   (address *("," address)) / obs-addr-list
   group-list      =   mailbox-list / CFWS / obs-group-list

type Address struct {
        Name    string // Proper name; may be empty.
        Address string // user@domain
}

type SingleAddress struct {
        Name    string // Proper name; may be empty.
        Address string // user@domain
}

type GroupAddress struct {
        Name string // group name
        Addresses []SingleAddress
}

type Address struct {
       SingleAddress
       IsGroup bool // true if address is group one
       GroupAddress GroupAddress
}