If you are using BPOS/Office365 and have a need to have 1 distribution group that includes 2 separate domains (ie: firstname.lastname@example.org and email@example.com), you may have seen that it causes a bit of a problem - you get an error message that the 'email address already exists'.
Here's the fix.
Name your FIRST distribution group firstname.lastname@example.org and include your members. Create your second distribution group email@example.com and include your members. Then go back to the original distribution group and change the address to firstname.lastname@example.org.
BPOS will now accept it!