write

WRITE  -  write or rewrite a mail file

Usage:  WRITE [filename]

Use the WRITE command with a filename to make a copy of your current mail 
  file, or without a filename, to rewrite the current mail file (thereby
  updating it).  WRITE does not remove deleted messages.

If you do specify a file without a directory, WRITE uses the MAIL-DIRECTORY, 
  which is by default your home directory.

The QUIT, EXIT, BYE, SUSPEND commands all do a WRITE to update the current
  mail file, as you leave MM.  Some other MM operations or conditions also
  also rewrite or may rewrite the current mail file.  Therefore you may 
  never need to use WRITE.

For more help type "help" and one of these topics:
  copy   move  filing   suspend