Following the Howto I've installed Courier.
Messaged returned to my sending account is:
The mail system
<firstname.lastname@example.org>: user unknown. Command output: WARN: quota string
'2097152' not parseable
Final-Recipient: rfc822; email@example.com
Diagnostic-Code: x-unix; WARN: quota string '2097152' not parseable
Can you tell what I've done wrong this time. Thanks again.