Shared mailbox with the same email (local part) can't be created to a different domain

Moi Matti,

The problem is that when you create a shared mailbox, the alias with the local name is also created. Thus, regardless of the domain part, you have the error you described.

Luckily the workaround is quite simple. Just create a shared mailbox with unique local name, such as support_domain2@domain2.com. After it is created, edit the mailbox email addresses to change the default address to support@domain2.com and you're done!