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Normally messages are printed in the foreground. If this is slow on
your system, you may elect to set
nil to print in the background. If you do this, do not delete
the message until it is printed or else the output may be truncated.
mh-lpr-command-format controls how the printing is
actually done. The string can contain one escape, `
%s', which is
filled with the name of the folder and the message number and is useful
for print job names. As an example, the default is `
"lpr -J '%s'"'.