Lotus Notes allows access to email accounts/databases to be delegated. Delegation can be to users or groups and it works in a granular way (ie read only, read write, full access etc). This is extremely useful both for creating shared mailboxes for interest groups and best of all for making sure your emails can be answered when you are on holiday without scribbling passwords down on a yellow sticky for a colleague. The owner of the mail database can initiate the delegation and email a shortcut to lucky recipients.
What is the Zimbra take on my favourite feature in Lotus Notes?