root:    hostmaster@my.domain

mydomain = my.domain
myorigin = $mydomain
alias_database = hash:/etc/aliases
relay_domains = my.domain
append_at_myorigin = no
...

Dec 22 16:45:19 myserver postfix/pickup[21509]: 5CFBA2011E0: uid=995  rom=<munin>
Dec 22 16:45:19 myserver postfix/cleanup[22094]: 5CFBA2011E0:
    message-id=<20151222154519.5CFBA2011E0@my.domain>
Dec 22 16:45:19 myserver postfix/qmgr[21510]: 5CFBA2011E0: from=<munin>, size=999, nrcpt=1
    (queue active)
Dec 22 16:45:20 myserver postfix/smtp[22099]: 5CFBA2011E0: to=<root@my.domain>,
    orig_to=<root>, relay=internal.my.domain[XXX.182.189.136]:25, delay=4.2,
    delays=3/0.01/1.1/0.09, dsn=5.1.1, status=bounced (host
    internal.my.domain[XXX.182.189.136] said: 550 5.1.1 <root@my.domain>: Recipient address
    rejected: User unknown in virtual mailbox table (in reply to RCPT TO command))
Dec 22 16:45:20 myserver postfix/cleanup[22094]: A98B72012D2:
    message-id=<20151222154520.A98B72012D2@my.domain>
Dec 22 16:45:20 myserver postfix/bounce[22100]: 5CFBA2011E0: sender non-delivery
    notification: A98B72012D2