使用re.search
以下模式:
>>> s = 'from mail2.oknotify2.com (mail2.oknotify2.com. [208.83.243.70]) by mx.google.com with ESMTP id dp5si2596299pdb.170.2015.06.03.14.12.03'
>>> import re
>>> re.search(r'\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}', s).group()
'208.83.243.70'