The handling of the -f option is a hack.  Instead of fooling around
with argv behind getopt()'s back we can just treat the remainder
of argv[] after option processing as the file name for -f.  It is
not possible to use -f in sending mode so there is no ambiguity.

Index: usr.bin/mail/main.c
===================================================================
--- usr.bin/mail/main.c.orig
+++ usr.bin/mail/main.c
@@ -111,16 +111,10 @@ main(int argc, char **argv)
 			/*
 			 * User is specifying file to "edit" with Mail,
 			 * as opposed to reading system mailbox.
-			 * If no argument is given after -f, we read his
-			 * mbox file.
-			 *
-			 * getopt() can't handle optional arguments, so here
-			 * is an ugly hack to get around it.
-			 */
-			if ((argv[optind]) && (argv[optind][0] != '-'))
-				ef = argv[optind++];
-			else
-				ef = "&";
+			 * We read his mbox file unless another file
+			 * is specified after the arguments.
+			 */
+			ef = "&";
 			break;
 		case 'n':
 			/*
@@ -169,17 +163,24 @@ main(int argc, char **argv)
 			/*NOTREACHED*/
 		}
 	}
-	for (i = optind; (argv[i]) && (*argv[i] != '-'); i++)
-		to = cat(to, nalloc(argv[i], GTO));
-	for (; argv[i]; i++)
-		smopts = cat(smopts, nalloc(argv[i], 0));
+	if (ef != NULL) {
+		/* Check for optional mailbox file name. */
+		if (optind < argc) {
+			ef = argv[optind++];
+			if (optind < argc)
+			    errx(1, "Cannot give -f and people to send to");
+		}
+	} else {
+		for (i = optind; (argv[i]) && (*argv[i] != '-'); i++)
+			to = cat(to, nalloc(argv[i], GTO));
+		for (; argv[i]; i++)
+			smopts = cat(smopts, nalloc(argv[i], 0));
+	}
 	/*
 	 * Check for inconsistent arguments.
 	 */
 	if (to == NULL && (subject != NULL || cc != NULL || bcc != NULL))
 		errx(1, "You must specify direct recipients with -s, -c, or -b");
-	if (ef != NULL && to != NULL)
-		errx(1, "Cannot give -f and people to send to");
 	/*
 	 * Block SIGINT except where we install an explicit handler for it.
 	 */
Index: usr.bin/mail/mail.1
===================================================================
--- usr.bin/mail/mail.1.orig
+++ usr.bin/mail/mail.1
@@ -79,11 +79,14 @@ to output all sorts of information usefu
 .Nm mail .
 .It Fl E
 Don't send messages with an empty body.
-.It Fl f Op Ar file
-Read in the contents of your mailbox
-(or the specified
-.Ar file )
-for processing; when you quit,
+.It Fl f
+Use an alternate mailbox.
+Defaults to the user's
+.Ar mbox
+if no
+.Ar file
+is specified.
+When quit,
 .Nm mail
 writes undeleted messages back to this
 .Ar file .