I've raised this with support a couple of times, but it's causing serious issues for us that we cannot set the same email address to multiple 'client' accounts that we have.
We deal with a number of marketing agencies and they in turn have clients with differnet lists, templates, services etc. etc. But some of our contacts require access to numerous clients.
We used to be able to set a master username/password for each client account - what happened to this? ..please can we have it back!
This is certainly a popular request. Watch this space - the ability to have a single email address access multiple clients is something we're looking very closely at.
As Phil suggested, we were indeed looking at adding the ability to have a single email address associated with more than one client.
This morning we've added that ability, so you can now add an existing address to as many clients as you like (with different permissions each time too, if you wish). I hope that makes things easier for you!