diff --git a/git-am.sh b/git-am.sh
index d64d997..f719f6e 100755
--- a/git-am.sh
+++ b/git-am.sh
@@ -191,6 +191,20 @@
 			esac
 			;;
 		esac
+		if test -z "$patch_format" &&
+			test -n "$l1" &&
+			test -n "$l2" &&
+			test -n "$l3"
+		then
+			# This begins with three non-empty lines.  Is this a
+			# piece of e-mail a-la RFC2822?  Grab all the headers,
+			# discarding the indented remainder of folded lines,
+			# and see if it looks like that they all begin with the
+			# header field names...
+			sed -n -e '/^$/q' -e '/^[ 	]/d' -e p "$1" |
+			egrep -v '^[A-Za-z]+(-[A-Za-z]+)*:' >/dev/null ||
+			patch_format=mbox
+		fi
 	} < "$1" || clean_abort
 }
 
@@ -254,7 +268,11 @@
 		msgnum=
 		;;
 	*)
-		clean_abort "Patch format $patch_format is not supported."
+		if test -n "$parse_patch" ; then
+			clean_abort "Patch format $patch_format is not supported."
+		else
+			clean_abort "Patch format detection failed."
+		fi
 		;;
 	esac
 }
diff --git a/t/t4150-am.sh b/t/t4150-am.sh
index a12bf84..8296605 100755
--- a/t/t4150-am.sh
+++ b/t/t4150-am.sh
@@ -77,6 +77,12 @@
 	git commit -s -F msg &&
 	git tag second &&
 	git format-patch --stdout first >patch1 &&
+	{
+		echo "X-Fake-Field: Line One" &&
+		echo "X-Fake-Field: Line Two" &&
+		echo "X-Fake-Field: Line Three" &&
+		git format-patch --stdout first | sed -e "1d"
+	} > patch1.eml &&
 	sed -n -e "3,\$p" msg >file &&
 	git add file &&
 	test_tick &&
@@ -108,6 +114,15 @@
 	test "$(git rev-parse second^)" = "$(git rev-parse HEAD^)"
 '
 
+test_expect_success 'am applies patch e-mail not in a mbox' '
+	git checkout first &&
+	git am patch1.eml &&
+	! test -d .git/rebase-apply &&
+	test -z "$(git diff second)" &&
+	test "$(git rev-parse second)" = "$(git rev-parse HEAD)" &&
+	test "$(git rev-parse second^)" = "$(git rev-parse HEAD^)"
+'
+
 GIT_AUTHOR_NAME="Another Thor"
 GIT_AUTHOR_EMAIL="a.thor@example.com"
 GIT_COMMITTER_NAME="Co M Miter"