When "e"diting a MBOX message mail(1) will place the entire message in the editor, i.e., including the separating newline.
If that newline is removed the edited message will be joined with the message before it.
(The good news is that this can be reversed just the same way.)

Fix: I've fixed that for S-nail(1) like follows, though it is hacky yet.
This patch will most likely not apply to FreeBSD mail(1).



/*
 	 * Deal with each message to be edited . . .
@@ -114,25 +111,35 @@ edit1(int *msgvec, int type)
 		did_print_dot = 1;
 		touch(mp);
 		sigint = safe_signal(SIGINT, SIG_IGN);
-		fp = run_editor(fp, mp->m_size, type,
+		--mp->m_size; /* XXX[edithack] strip final NL */
+		fp = run_editor(fp, -1/*mp->m_size*/, type,
 				(mb.mb_perm & MB_EDIT) == 0 || !wb,
 				NULL, mp, wb ? SEND_MBOX : SEND_TODISP_ALL,
 				sigint);
+		++mp->m_size; /* XXX[edithack] */
 		if (fp != NULL) {
 			fseek(mb.mb_otf, 0L, SEEK_END);
 			size = ftell(mb.mb_otf);
 			mp->m_block = mailx_blockof(size);
 			mp->m_offset = mailx_offsetof(size);
-			mp->m_size = fsize(fp);
 			mp->m_lines = 0;
 			mp->m_flag |= MODIFY;
 			rewind(fp);
+			lastnl = 0;
+			size = 0;
 			while ((c = getc(fp)) != EOF) {
-				if (c == '\n')
+				if ((lastnl = c == '\n'))
 					mp->m_lines++;
 				if (putc(c, mb.mb_otf) == EOF)
 					break;
+				++size;
 			}
+			/* MBOX finalize XXX[edithack] is this always MBOX? */
+			if (! lastnl && putc('\n', mb.mb_otf) != EOF)
+				++size;
+			if (putc('\n', mb.mb_otf) != EOF)
+				++size;
+			mp->m_size = (size_t)size;/*XXX[edithack] inc.MBOX?!? */
 			if (ferror(mb.mb_otf))--NgsVrfL9B098AsGfSU2RUXXt7iK1sF0Ln724AuHmMIZES6ki
Content-Type: text/plain; name="file.diff"
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="file.diff"

--- /Users/steffen/tmp/f.new	2012-10-24 23:50:48.000000000 +0200
+++ /Users/steffen/tmp/f.old	2012-10-24 23:50:34.000000000 +0200
@@ -80,14 +80,12 @@ visual(void *v)
 static int 
 edit1(int *msgvec, int type)
 {
-	int c;
-	int i;
+	int c, i, wb, lastnl;
 	FILE *fp = NULL;
 	struct message *mp;
 	off_t size;
 	char *line = NULL;
 	size_t linesize;
-	int	wb;
How-To-Repeat: mail -f the following MBOX and "e"dit the first message by just stripping off the final newline.


From S-Postman Thu May 10 20:40:54 2012
From: <1234567890@abc.com>
To: <recei@ver.com>
Subject: Example mail
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Status: RO


I agree it has nothing to do with tz, so wouldn't it have been more conside=
rate not to send it?


From S-Postman Thu May 10 20:40:54 2012
From: <1234567890@abc.com>
To: <recei2@ver2.com>
Subject: Example mail2
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Status: O


I agree it has nothing to do with tz, so wouldn't it have been more conside=
rate not to send it?