Originally posted by Tochjo & CookieRevised:

Block checkers do not work.
A long time ago, there was a bug in the protocol that allowed these block checkers to see what someone's real status was. If the status was online, and the contact displayed as offline, then you would know that you were blocked; it did not check if you were 'blocked', it merely checked the status you had set.

Now there is an option to only allow people on your allow list to see your status and send you messages. If this option is on, block checkers will always detect that contact as being offline, even if he/she is in fact online: simply because the block checker is not on your allow list.

When we say the bug has been fixed, we mean that block checkers could get past that option a long time ago and still show your real status, even if they were not on the allow list, but now that has been fixed, they can't get past it anymore.

Everyone can easily verify this for themselves: check the box (it's on the Privacy tab at the Options you can set in the menu Tools) and use block checkers: make sure your status is set to online and enter your own email address.

You will see it shows you as offline, though you know you are online 

As for existing online "blockcheckers":

1) There is no need to know your name. Block checkers asking for this are utterly fake.
2) There is certainly NO need to know your own email address. Block checkers requiring this are completely fake and nothing more than email gathering tools from spammers!!
3) Certainly do not enter your Windows Live ID password in online block checkers. Such block checkers are simply made to steal your password!
4) If a connection can not be made by such block checkers, they should NEVER say you are blocked; that is completely wrong information.
5) There was only 1 way that a block checker could work. Any block checker claiming it has found another way and thus requires other info from you (eg: password, your email, etc) are completely fake and are malicious tools to steal your data.
6) Really block checking is not possible anymore since a long time

90% of all block checkers which still can be found are email gathering tools (yes, emails ARE logged to be sold to spammers). And even if that '90%' isn't entirly correct, the small chance that even 1 block checker out there is used to gather emails, makes that block checkers should NEVER be used. You wouldn't want your email address to be listed on spam lists, so do NOT enter other people's email address in such block checker forms either... They do not work anyways!