Fizl

Итак, спасибо за помощь всем !
Вот что вышло :

import tkinter as tk
from tkinter import ttk
from functools import partial
import smtplib
from email.mime.text import MIMEText
from email.header import Header
import codecs
import email
import imaplib
from threading import Thread
import threading
from tkinter import messagebox
from email.header import decode_header
import codecs
import time
import os

class MAIN_WIND(tk.Frame):
    def __init__(self, main_root):
        super().__init__(main_root)
        self.contact_name = "TXT"
        self.MAIL = MAIL('youremail@gmail.com', smtplib.SMTP('smtp.gmail.com:587'))
        self.GET_MAIL = GET_MAIL(imaplib.IMAP4_SSL('imap.gmail.com'))
        self.init_main()
        threading.Thread(target = self.main, daemon = True).start()



    def follow(self, name):
        current = codecs.open(name, "r", encoding = "utf-8")
        curino = os.fstat(current.fileno()).st_ino
        while True:
            while True:
                line = current.readline()
                if not line:
                    break
                yield line

            try:
                if os.stat(name).st_ino != curino:
                    new = open(name, "r")
                    current.close()
                    current = new
                    curino = os.fstat(current.fileno()).st_ino
                    continue
            except IOError:
                pass
            time.sleep(1)

    def follow_2(self):
        if __name__ == '__main__':
            for l in self.follow(f"{self.contact_name}.txt"):
                #print(format(l))
                self.text_show.insert(1.0, l)

    def main(self):
        threading.Thread(target = self.follow_2, daemon = True).start()
        threading.Thread(target = self.GET_MAIL.get_mail_func, daemon=True).start()

    def retrieve_mail(self):
        self.retr_mail = self.text_enteremail.get(1.0, tk.END)
        self.MAIL.send_mail(self.retr_mail)

    def init_main(self):
        self.MAIL.start_client('smtp.gmail.com:587')

        self.btn_dialog_sendmail = tk.Button(main_root, text = '➤', padx = 8, pady = 24, font = ('Ubuntu',25), command = self.retrieve_mail)
        self.text_enteremail = tk.Text(main_root, width = 30, height = 6, bg='white', fg='black', font = ('COMIC SANS MS',10))

        #////SCROLLBAR////
        self.dialog_frame = tk.Frame(main_root)
        #///////////////Вот эта переменная/////////////////
        self.text_show = tk.Text(self.dialog_frame,width=40, height=24)
        #/////////////////////////////////////////////////
        self.scroll = tk.Scrollbar(self.dialog_frame, command=self.text_show.yview)
        self.scroll.pack(side=tk.RIGHT, fill=tk.Y)
        self.text_show.config(yscrollcommand=self.scroll.set)
        #self.text_show.configure(state='disabled')
        self.text_show.pack(side=tk.TOP) 
        self.dialog_frame.pack(side=tk.TOP)
        #////END SCROLLBAR////

        self.btn_dialog_sendmail.pack()
        self.text_enteremail.pack()

        self.btn_dialog_sendmail.place(x = 279,y = 400)
        self.text_enteremail.place(x = 29, y = 400)

Нужно было перенести init_main() выше потока и убрать строку "self.text_show.configure(state='disabled')". Еще для удобства (вроде бы необязательно) можно запустить только один поток, внутри которого работают два потока))