>>> import random
>>> 
>>> def gen():
...     MINLEN, MAXLEN = 5, 10
...     domains = ('gmail.com', 'yandex.ru', 'mail.ru')
...     alpha = ('abcdefghijklmnopqrstuvwxyz'
...              'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
...              '0123456789')
...     login_len = random.randint(MINLEN, MAXLEN)
...     login = ''.join(random.choice(alpha) for _ in range(login_len))
...     out = '@'.join((login, random.choice(domains)))
...     return out
... 
>>> gen()
'uXa0Nvj@yandex.ru'
>>> gen()
'uibYB3@mail.ru'
>>> gen()
'0S4wJzfeEk@yandex.ru'
>>>