Postfix Overview - Introduction - SCN Research

Postfix Configuration - Resource Controls
mail delivery requests when there is insufficient space (the mail will be accepted once enough space
becomes available). There is no default limit; however, it seems like a good idea to require at least several
times $message_size_limit so that the mail system won't get stuck on a single large message.
bounce_size_limit (default: 50000 bytes)
How much of an undelivered message is sent back to the sender.
Object count limits
Once the sizes of memory objects have been limited, the next step to implement Postfix's finite memory budget
is to limit the number of in-memory object instances.
qmgr_message_recipient_limit (default: 10000)
An upper bound on the number of queue manager in-memory recipient address data structures. This
parameter also controls the number of instances of other in-memory data structures. See, for example, the
delivery rate control documentation.
qmgr_message_active_limit (default: 1000)
An upper limit on the number of messages in the active queue. For an introduction to the Postfix queue
organization see the Postfix overview documentation.
duplicate_filter_limit (default: 1000)
How many recipient addresses the local delivery agent and address cleanup daemon remember when
delivering a message. A recipient address is ignored when it is found in the remembered list.
Time limits
External commands are given a finite time for completion. Such commands are run by the local delivery agent
when it finds a "|command" destination in an alias database, :include: file or .forward file. The pipe mailer
implements an alternative way to pipe mail into external commands.
command_time_limit (default: 1000 seconds)
How long the local delivery agent will wait before aborting an external command.
service_name_time_limit (default: $command_time_limit)
The time limit for delivery to external commands via the pipe mailer. For service_name, substitute the
service name (the first field in the master.cf file).
Acquiring exclusive file locks
Internally, the Postfix programs cooperate in a very disciplined manner and rarely need to fight for exclusive
file access. However, access conflicts may happen on the outside, for example, when mail has to be delivered
while a user is accessing her mailbox. Postfix supports two types of file locks:
• Internal locks, implemented with the fcntl() or flock() system primitives.
• External locks, implemented as files named file.lock.
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Depending on the host system, Postfix uses one method or both. The following configuration parameters control
how Postfix deals with file locks:
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