unsub.py

from typing import List

def get_mails(path: str, limit: int = None) -> List[str]:
    '''Extract emails from an .mbox file.'''
    mails = []
    c = -1
    mail = str()
    with open(path, 'r', encoding = 'UTF-8') as file:
        for line in file:               #read every line
            if c == limit: break
            if line.startswith('From'): #when a new mail starts...
                c += 1                  #add it to the counter
                mails.append(mail)      #add the lines that you've been saving
                mail = line             #reset the mail text to the first line
            else:
                mail += line            #add each line to the mail text
    return mails[1:]                    #first line is redudant