Thanks to Filip Van Canneyt for pointing us to this interesting information

Office 365 has a really nice Lync Online feature. They say that you can federate your Lync Online to Lync deployments at other companies. Well, that is only partially true.

The way Lync works today is that if I have Lync configured in my organization and have an Edge environment that is configured for open federation, then I can easily connect to and communicate with my business partners in other companies that are also configured for open federation. It is a great idea, and it saves a ton of money and effort when communicating with others in outside organizations.

Well, Lync Online doesn’t work that way. If I, as a Lync Online user, want to communicate via Lync with my business partners that also use Lync, then they have to do way more than just configure open federation. They have to specifically add the Lync Online environment because Microsoft doesn’t seem to know how to implement their own product.

Read the full article and how to solve it here:
http://infrastructurehelp.wordpress.com/2011/03/15/office-365a-serious-flaw-with-lync/