fix(GODT-1623): Throttle SMTP failed requests

If a SMPT client keeps hammering bridge and triggers multiple successive
errors in quick succession, force that client to wait 20 seconds before
trying again.

internal/services/smtp/accounts_test.go

@@ -0,0 +1,46 @@
+// Copyright (c) 2023 Proton AG
+//
+// This file is part of Proton Mail Bridge.
+//
+// Proton Mail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Proton Mail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Proton Mail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package smtp
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAccountTimeout(t *testing.T) {
+	account := smtpAccountState{errTimeout: 5 * time.Second}
+	err := errors.New("fail")
+
+	for i := 0; i <= maxFailedCommands; i++ {
+		requestTime := time.Now()
+		assert.Nil(t, account.canMakeRequest(requestTime))
+		account.handleSMTPErr(requestTime, err)
+	}
+	{
+		requestTime := time.Now()
+		assert.ErrorIs(t, account.canMakeRequest(requestTime), ErrTooManyErrors)
+	}
+
+	assert.Eventually(t, func() bool {
+		requestTime := time.Now()
+		return account.canMakeRequest(requestTime) == nil
+	}, 10*time.Second, time.Second)
+}