What does failed to create child process to send failure message mean? This is a busy mail server with smtp_accept_max set to 500, but this problem started to occur at about 300 incoming connections.

Some message delivery failed, and when Exim wanted to send a bounce message, it was unable to create a process in which to do so. Probably the limit on the maximum number of simultaneously active processes has been reached. Most OS have some means of increasing this limit, and in some operating systems there is also a limit per uid which can be varied.

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