A random question regarding owl post

[Cassandra Claire]

No: HPFGUIDX 3044

Hmm. I was working on a fanfic story and a question occured to me 
regarding owl post...in order to send a letter via owl, in the canon, 
is it necessary to have some kind of address to send the owl to? I 
know this may seem like a silly question, but IIRC, Harry just tells 
Hedwig "take this letter to Sirius" or some such, and the owl flies 
off and delivers it. Harry doesn't even know exactly where Sirius is. 
I suppose the question is: if you don't know precisely where someone 
is, can you still send them a letter via owl?