If you want to block someone else from sending you e-mail it is relatively simple to do so. Simply login to your Yahoo e-mail account and click on Options, then Mail Options. Then on the list on the left-hand side of the screen click on Spam. This will show the blocked address box. Enter the e-mail address of the user you want to block and click Add Block. You must finally make sure you save this added block by clicking on Save Changes, which is at the top of the screen.
You can check if somebody has read your e-mails by registering on one of the following websites: "www.readnotify.com" or "www.didtheyreadit.com." Registering with one of these websites will enable you to receive an automated message when somebody opens an e-mail you have sent them, but it won't send you a message if they have blocked you.
Yahoo is not the only mail provider that you are unable to check on blocked accounts with. Similarly, Google provides no way of know of identifying whether or not somebody has placed a block on e-mails you are sending them.
There is another reason that somebody has not received your e-mails. They may have a very strong spam filter, so it would be advisable to check this with them before sending them e-mails if this is possible.