add per-account quota for total message size disk usage

so a single user cannot fill up the disk.
by default, there is (still) no limit. a default can be set in the config file
for all accounts, and a per-account max size can be set that would override any
global setting.

this does not take into account disk usage of the index database. and also not
of any file system overhead.

smtpserver/server_test.go

@@ -1240,6 +1240,37 @@ func TestLimitOutgoing(t *testing.T) {
 	testSubmit("b@other.example", &smtpclient.Error{Code: smtp.C451LocalErr, Secode: smtp.SePol7DeliveryUnauth1}) // Would be 5th message.
 }
 
+// Test account size limit enforcement.
+func TestQuota(t *testing.T) {
+	resolver := dns.MockResolver{
+		A: map[string][]string{
+			"other.example.": {"127.0.0.10"}, // For mx check.
+		},
+		PTR: map[string][]string{
+			"127.0.0.10": {"other.example."},
+		},
+	}
+	ts := newTestServer(t, filepath.FromSlash("../testdata/smtpserverquota/mox.conf"), resolver)
+	defer ts.close()
+
+	testDeliver := func(rcptTo string, expErr *smtpclient.Error) {
+		t.Helper()
+		ts.run(func(err error, client *smtpclient.Client) {
+			t.Helper()
+			mailFrom := "mjl@other.example"
+			if err == nil {
+				err = client.Deliver(ctxbg, mailFrom, rcptTo, int64(len(deliverMessage)), strings.NewReader(deliverMessage), false, false, false)
+			}
+			var cerr smtpclient.Error
+			if expErr == nil && err != nil || expErr != nil && (err == nil || !errors.As(err, &cerr) || cerr.Secode != expErr.Secode) {
+				t.Fatalf("got err %#v, expected %#v", err, expErr)
+			}
+		})
+	}
+
+	testDeliver("mjl@mox.example", &smtpclient.Error{Code: smtp.C452StorageFull, Secode: smtp.SeMailbox2Full2})
+}
+
 // Test with catchall destination address.
 func TestCatchall(t *testing.T) {
 	resolver := dns.MockResolver{