>>> def check_log(login):
...     """проверяем логин, если норм возвращаем True"""
...     return True
... 
>>> def check_pass(password):
...     """проверка"""
...     return True
... 
>>> txt = """\
... login:password
... asdasd-!asd1321@   
... login2:password2
... login3:password3
... login+password
... 
... +login6:-:password:
... login7::+:password
... login8:-:password:
... 
... login4:password4
... login5:password5\
... """
>>> 
>>> out = ''
>>> 
>>> for line in txt.splitlines():
...     if line.count(':') == 1:
...         login, password = line.split(':')
...         if check_log(login) and check_pass(password):
...             out += '{}:{}\n'.format(login, password)
... 
>>> print(out)
login:password
login2:password2
login3:password3
login4:password4
login5:password5
 
>>>