How do I change postmaster email?

==== Required information ====
- iRedMail version: 0.8.4
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Linux/BSD distribution name and version: Ubuntu 13.04 Raring 32bit
- Related log if you're reporting an issue:
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Hi,

    I'm just wondering how do I change the postmaster email name?  I would like it to be something else.

    Thank you in advance