Thank you - you're right, of course, that it's very unlikely it actually disappears without a trace, even with forks and execs. :) I've been checking every logfile I could find and even been rgrepping through other possible locations without any result. I do find qmail logs, but nothing from this.

However, the idea to insert debugging code into _fail did give a result:

81 53 Error closing write pipe: Broken pipe

Reading up further on the source in Send.pm, this message comes from after the body is written and the pipe is supposed to be closed to signal that it's time for the next pipe, the envelope, to be written. The closing is what fails, or at least triggers the fail in the script.

Now this is sadly where I'm stumped again. I do not really know how to proceed from here, I don't really know anough about this kind of communication to see why this may be...

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