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message_size_limit
(default: 10240000)
The maximal size in bytes of a message, including envelope information.
Note: be careful when making changes. Excessively small values
will result in the loss of non-delivery notifications, when a bounce
message size exceeds the local or remote MTA's message size limit.
queue_directory
(default: see "postconf -d" output)
The location of the Postfix top-level queue directory. This is the
root directory of Postfix daemon processes that run chrooted.
queue_file_attribute_count_limit
(default: 100)
The maximal number of (name=value) attributes that may be stored
in a Postfix queue file. The limit is enforced by the cleanup(8)
server.
This feature is available in Postfix 2.0 and later.
queue_minfree
(default: 0)
The minimal amount of free space in bytes in the queue file system
that is needed to receive mail. This is currently used by the
Postfix SMTP server to decide if it will accept any mail at all.
By default, the Postfix SMTP server rejects MAIL FROM commands when
the amount of free space is less than 1.5*$message_size_limit
(Postfix version 2.1 and later).
To specify a higher minimum free space limit, specify a queue_minfree
value that is at least 1.5*$message_size_limit.
With Postfix versions 2.0 and earlier, a queue_minfree value of
zero means there is no minimum required amount of free space.
queue_run_delay
(default: 300s)
The time between deferred queue scans by the queue manager;
prior to Postfix 2.4 the default value was 1000s.
This parameter should be set less than or equal to
$minimal_backoff_time. See also $maximal_backoff_time.
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is s (seconds).
queue_service_name
(default: qmgr)
The name of the qmgr(8) service. This service manages the Postfix
queue and schedules delivery requests.
This feature is available in Postfix 2.0 and later.
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From all of above, I understand that if an email have 20 MB (message size), it can be delivered by the Postfix SMTP only if the Queue (directory) have more than 1.5*20=30 MB. Am I wrong?