Here are fixes to from and mail to use the value of $MAIL (if set)
as the default mailbox like SunOS does...

christos 

*** usr.bin/mail/v7.local.c.dist	Sun Nov  7 05:05:57 1993
--- usr.bin/mail/v7.local.c	Mon Jan  3 09:56:56 1994
***************
*** 45,50 ****
--- 45,51 ----
   */
  
  #include "rcv.h"
+ #include <stdlib.h>
  
  /*
   * Locate the user's mailbox file (ie, the place where new, unread
***************
*** 53,59 ****
  findmail(user, buf)
  	char *user, *buf;
  {
! 	(void)sprintf(buf, "%s/%s", _PATH_MAILDIR, user);
  }
  
  /*
--- 54,64 ----
  findmail(user, buf)
  	char *user, *buf;
  {
! 	char *mbox;
! 	if (!(mbox = getenv("MAIL")))
! 		(void)sprintf(buf, "%s/%s", _PATH_MAILDIR, user);
! 	else
! 		(void)strcpy(buf, mbox);
  }
  
  /*
*** usr.bin/from/from.c.dist	Sun Nov  7 05:03:27 1993
--- usr.bin/from/from.c	Mon Jan  3 09:51:22 1994
***************
*** 46,51 ****
--- 46,53 ----
  #include <ctype.h>
  #include <pwd.h>
  #include <stdio.h>
+ #include <stdlib.h>
+ #include <unistd.h>
  #include <paths.h>
  
  main(argc, argv)
***************
*** 82,101 ****
  		}
  	argv += optind;
  
  	if (!file) {
  		if (!(file = *argv)) {
! 			if (!(pwd = getpwuid(getuid()))) {
! 				fprintf(stderr,
! 				    "from: no password file entry for you.\n");
! 				exit(1);
  			}
- 			file = pwd->pw_name;
  		}
! 		(void)sprintf(buf, "%s/%s", _PATH_MAILDIR, file);
! 		file = buf;
  	}
  	if (!freopen(file, "r", stdin)) {
! 		fprintf(stderr, "from: can't read %s.\n", file);
  		exit(1);
  	}
  	for (newline = 1; fgets(buf, sizeof(buf), stdin);) {
--- 84,113 ----
  		}
  	argv += optind;
  
+ 	/*
+ 	 * We find the mailbox by:
+ 	 *	1 -f flag
+ 	 *	2 user
+ 	 *	2 MAIL environment variable
+ 	 *	3 _PATH_MAILDIR/file
+ 	 */
  	if (!file) {
  		if (!(file = *argv)) {
! 			if (!(file = getenv("MAIL"))) {
! 				if (!(pwd = getpwuid(getuid()))) {
! 					(void)fprintf(stderr,
! 				"from: no password file entry for you.\n");
! 					exit(1);
! 				}
! 				(void)sprintf(file = buf, "%s/%s",
! 					      _PATH_MAILDIR, pwd->pw_name);
  			}
  		}
! 		else
! 			(void)sprintf(file = buf, "%s/%s", _PATH_MAILDIR, file);
  	}
  	if (!freopen(file, "r", stdin)) {
! 		(void)fprintf(stderr, "from: can't read %s.\n", file);
  		exit(1);
  	}
  	for (newline = 1; fgets(buf, sizeof(buf), stdin);) {

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