You can give admin1 full access to user1 mailbox and allow him to send emails as user1. user1 can be shared mailbox such as Sales, Support, etc.

You can do this either in Exchange Management Console (EMC) or Exchange Management Shell (EMS)

Option 1: EMC

1. EMC > Recipient Configuration > Mailbox [use1] > Right Click > Manage Send As Permission > Add.
   Find admin1 > OK > Manage
2. EMC > Recipient Configuration > Mailbox [user1] > Right Click > Manage Full Access Permission > Add.
   Find admin1 > OK > Manage

Option 2: EMS

1. Add-MailboxPermission -identity user1 -User admin1 -AccessRights FullAccess

2. Add-ADPermission “User1 Name” -User admin1 -AccessRights extendedright -ExtendedRights “send as”
   “User1 Name” – full object’s name (or distinguished name), not alias.

After steps in either option are performed admin1 should immediately have access to user1 mailbox, however Send-As permission may not kick in for an hour or two.

Exchange 2007