I think you are confused by the uncorrected comments in document, actually there's no 'sender' type.
recipient_type t = r.get_type(); // get the type of the recipient, sender, to, cc, etc.
I'm confused at the beginning as well... I got sender like this:
if (m.get_property_bag().prop_exists(0x0c1a) // suppose m is a message.
sender = m.get_property_bag().read_prop<std::string>(0x0c1a);
The key is finding the tagId(0x0c1a) for the field you want, refer this link http://msdn.microsoft.com/en-us/library/cc433490%28EXCHG.80%29.aspx
Which time are you talking about?