For the EX installation you have to create a new user, called "service account". This points to the fact that his account is used to run the EX services.
To do this, you may of course use an existing account. However, to not confuse the system, it is good practice to create a new one. This user must have
- Local Admin rights to be able to install the EX software
- Domain Admin rights are also needed
- Exchange Admin rights, because it modifies the Exchange system
If you want to use this account to collect all mails, it must have the rights to access all mailboxes.