Address Public Private key verification Apologies if this question has been asked before, but when a bitcoin client wishes to send funds to an address, does the bitcoin protocol force the client to verify that the address has a corresponding private key? 

For example, I am given an address and I want to send a btc to this address, before I am allowed to authorize this transaction does the bitcoin protocol force me to verify that the address can decrypt a message encrypted from its public key?
No, there is no enforcement that a private key exists. 

However, due to a Bitcoin address including a checksum, it is very unlikely for the client to create a transaction when for an address with a typo, paste error, or other erroneous data.  There are addresses that will pass the checksum test in which, presumably, no private key exists. 1BitcoinEaterAddressDontSendf59kuE is, in all likelihood, one such address.