[CLUE-Tech] email in script

[Roger Frank]

I have a lot of programming lab exercises posted at
  http://www.rfrank.net/apcs/cslabs.html
and many teachers request solutions to those labs.
I want to return the solutions with a script.
I use:

  #!/bin/sh
  L=1120
  T=user at somewhere.net
  mutt -a "/var/local/aplabs/solution_$L.zip" \
     -s "Solution for Lab $L" $T \
      < "/var/local/aplabs/solution.txt"

and it works fine most of the time.  My ~/.muttrc has

  set realname="Roger Frank"
  set from="rfrank at rfrank.net"
  set use_from

so it goes out with a reasonable header.

Most of the time there is no problem.  But many times the 
mail bounces and the error is:

  <user at somewhere.net>: host mx1.somewhere.net[167.206.5.6] said: 550 5.1.8
    invalid/host-not-in-DNS return address not allowed

I've googled and I'm stumped so I'll turn to the real experts.
Your thoughts?

-- 
Roger Frank                                        rfrank at rfrank.net   
http://www.rfrank.net        Ponderosa High School, Parker, Colorado