from imaplib import IMAP4_SSL

YA_HOST = "imap.yandex.ru"
YA_PORT = 993
YA_USER = "LOGIN"
YA_PASSWORD = "PASSWROD"
SENDER = "Yoba"

connection = IMAP4_SSL(host=YA_HOST, port=YA_PORT)
connection.login(user=YA_USER, password=YA_PASSWORD)
status, msgs = connection.select('INBOX')
assert status == 'OK'

typ, data = connection.search(None, 'FROM', '"%s"' %s SENDER)
print(data)
for num in data[0].split():
    typ, message_data= connection.fetch(num, '(RFC822)')
    print(data)
    print('Message %sn%sn' % (num, message_data[0][1]))
connection.close()
connection.logout()

>>> [b'1757 1758 1759 1760 1774 1775']

mail = email.message_from_bytes(message_data[0][1])

if mail.is_multipart():
    for part in mail.walk():
        content_type = part.get_content_type()
        filename = part.get_filename()
        if filename:
            # Нам плохого не надо, в письме может быть всякое барахло
            with open(part.get_filename(), 'wb') as new_file:
                new_file.write(part.get_payload(decode=True))