TL;DR: programs that receive email from the Postfix pipe daemon should not produce large amounts of output.
When Postfix logs output from a pipe-to-command delivery, there are three output limits in effect: - The capacity of a UNIX pipe. The pipe daemon does not read any command output until AFTER it has written the entire email message to the command. That is one reason why the command should not produce lots of output. The number is platform dependent. It is not PIPE_BUF - that is an entirely different thing, concerned with atmoic writes. - The amount of command output that the pipe daemon will read before it closes the pipe. That's 4096 bytes, the Postfix internal buffer size. A program that tries to write more than that may experience a write error. - The size of logging output. Postfix limits the size to 2000 bytes. As for status 120, see https://bugs.python.org/issue5319, it appears to be related to stdout write errors in Python 3.6 and later. Wietse
