Sometimes you are reminded that you didn't document something you thought you did. Turns out that I wasn't even the one to write this up the first time but Gary did while he was on the team (since moved to ISA support).
As of October 11, 2005 all EASI* Passport domains are fully supported for the Public IM Connectivity (PIC) Service. We also refer to these as Vanity domains in case you hear the term. This means that Microsoft Office Communicator users can communicate with MSN users using EASI Passport IDs through PIC. EASI ID allows MSN users to create an IM address through Passport using the domain name of their choice. For example a user’s EASI Passport ID could have been email@example.com <mailto:firstname.lastname@example.org>. This is a great service for MSN users but this caused problems once PIC enabled users tried to communicate with them through Office Communicator. With this change, the EASI Passport ID can be used with PIC by making a simple change when adding a user to your Communicator contact list. A Communicator user that wants to add the EASI Passport ID of email@example.com would simply add the following address to their search field: username(contoso.com)@msn.com This increased the effectiveness of the PIC service as you now no longer need contacts with unsupported EASI Passport domains, to change their domains in order to use the PIC Service.
EASI = e-mail name as sign-in name
So MSN/Live/Hotmail offers alternate domain names for corporate use (sometimes referred to as vanity
Morten sent a question via the comments about a problem with R2 and trying to add PIC users with EASI