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Postfix Documentation
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Simple content filter limitations

The problem with content filters like the one above is that they are not very robust. The reason is that the software does not talk a well-defined protocol with Postfix. If the filter shell script aborts because the shell runs into some memory allocation problem, the script will not produce a nice exit status as defined in the file /usr/include/sysexits.h. Instead of going to the deferred queue, mail will bounce. The same lack of robustness can happen when the content filtering software itself runs into a resource problem.

The simple content filter method is not suitable for content filter actions that are invoked via header_checks or body_checks patterns. These patterns will be applied again after mail is re-injected with the Postfix sendmail command, resulting in a mail filtering loop. The advanced content filtering method (see below) makes it possible to turn off header_checks or body_checks patterns for filtered mail.

Postfix Documentation
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