OT mail system configuration

[Dave Howorth <dhoworth@mrc-lmb.cam.ac.uk>]

I use a mail forwarding address at the ACM.  There's a problem with 
their mail system in that when they have temporary systems problem their 
mail system returns a permanent 550 5.1.1 error to the sender. They 
claim they can't fix the problem!

I'm not a mail expert, so I'm hoping that somebody may be able to tell 
me what mail system they're using (some headers below) and confirm 
whether or not it can be configured correctly.  Also, can anybody tell 
me the name of a mail system on Debian that would be a suitable replacement?

Thanks, Dave

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