When your e-mail provider's server sends mail to another server, the receiving server sends an acknowledgement back, to confirm receipt. If no acknowledgement is received, your server keeps sending the mail, until it eventually gives up. At this point, your e-mail provider's server sends you a bounced mail message. (That's actually slightly simplified. In reality, there are usually several servers along the route. It's the last sending server which would bounce the mail if the intended recipient server failed to acknowledge it).
So, in theory, you should always get a 'bounced mail' message if your mail can't reach the server at the end of the chain. (In practice, there can occasionally be problems with the 'bounce' message finding its way back to you).
However, the fact that an e-mail can arrive at an Inbox might mean that the address is 'valid' but that's not the same as saying that the account is 'active'. For example, if I give out an e-mail address on the AnswerBank, it's usually one I've set up just for that one person to contact me. Once our correspondence is at an end, the address will remain valid (in that any mail sent to it will arrive at my Inbox on the host server, and not be bounced) but I delete the account from my PC and never connect to it again. So there are at least one hundred old e-mail addresses (which I've used in the past) which are still 'valid' but they're certainly not 'active' because I never access them. (The mail just sits on the server forever).
You can attempt to see if your mail has got through by clicking on 'Tools' and selecting 'Request Read Receipt', when composing mail in Outlook Express. However, it's up to the recipient to decide whether to allow his PC to send you a receipt.
Chris