I have several users that need access to a common mailbox, lets call it email@example.com. Each of the users have an AD account in domain.com, and have their own Exchange mailboxes. After doing some research, delegated mailboxes were ruled out because sent and deleted mail is kept in the delegate's folders, not the original mailbox's. I found that Outlook 2010 can have "Additional Accounts." Here's a good overview of those. The nice thing about the additional account is that it behaves identical to the users original mailbox (signatures, sent mail kept in sent folder, deleted mail kept in trash), and that the "from" address can be swapped quickly when writing an email.
I set up the users with the firstname.lastname@example.org mailbox, but have been having trouble since doing so. The first issue is that after adding the additional account, a credentials box pops up every time after opening Outlook. The user types in the user name as email@example.com, enters the password and checks for the credentials to be remembered. This worked fine for about a week. Now, some of the users are no longer able to send mail from the firstname.lastname@example.org account. A message pops up saying that they need "Send-As" permission. I don't understand why they would need permissions since they are logging in as that mailbox and should be treated as "owners." I've followed through several technet forum posts and tried several solutions, but nothing has worked yet.