diff --git a/client_test.go b/client_test.go
index d1afc86..2ac8811 100644
--- a/client_test.go
+++ b/client_test.go
@@ -17,6 +17,7 @@ import (
 	"net/mail"
 	"os"
 	"reflect"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -34,14 +35,15 @@ const (
 	TestServerProto = "tcp"
 	// TestServerAddr is the address the simple SMTP test server listens on
 	TestServerAddr = "127.0.0.1"
-	// TestServerPortBase is the base port for the simple SMTP test server
-	TestServerPortBase = 30025
 	// TestSenderValid is a test sender email address considered valid for sending test emails.
 	TestSenderValid = "valid-from@domain.tld"
 	// TestRcptValid is a test recipient email address considered valid for sending test emails.
 	TestRcptValid = "valid-to@domain.tld"
 )
 
+// TestServerPortBase is the base port for the simple SMTP test server
+var TestServerPortBase int32 = 30025
+
 // PortAdder is an atomic counter used to increment port numbers for the test SMTP server instances.
 var PortAdder atomic.Int32
 
@@ -98,6 +100,18 @@ type logData struct {
 	Lines []logLine `json:"lines"`
 }
 
+func init() {
+	testPort := os.Getenv("TEST_BASEPORT")
+	if testPort == "" {
+		return
+	}
+	if port, err := strconv.Atoi(testPort); err == nil {
+		if port <= 65000 && port > 1023 {
+			TestServerPortBase = int32(port)
+		}
+	}
+}
+
 func TestNewClient(t *testing.T) {
 	t.Run("create new Client", func(t *testing.T) {
 		client, err := NewClient(DefaultHost)
diff --git a/smtp/smtp_test.go b/smtp/smtp_test.go
index 1b616fc..764d65b 100644
--- a/smtp/smtp_test.go
+++ b/smtp/smtp_test.go
@@ -30,6 +30,7 @@ import (
 	"io"
 	"net"
 	"os"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -46,13 +47,14 @@ const (
 	TestServerProto = "tcp"
 	// TestServerAddr is the address the simple SMTP test server listens on
 	TestServerAddr = "127.0.0.1"
-	// TestServerPortBase is the base port for the simple SMTP test server
-	TestServerPortBase = 30025
 )
 
 // PortAdder is an atomic counter used to increment port numbers for the test SMTP server instances.
 var PortAdder atomic.Int32
 
+// TestServerPortBase is the base port for the simple SMTP test server
+var TestServerPortBase int32 = 30025
+
 // localhostCert is a PEM-encoded TLS cert generated from src/crypto/tls:
 //
 //	go run generate_cert.go --rsa-bits 1024 --host 127.0.0.1,::1,example.com \
@@ -231,6 +233,18 @@ var authTests = []authTest{
 	},
 }
 
+func init() {
+	testPort := os.Getenv("TEST_BASEPORT")
+	if testPort == "" {
+		return
+	}
+	if port, err := strconv.Atoi(testPort); err == nil {
+		if port <= 65000 && port > 1023 {
+			TestServerPortBase = int32(port)
+		}
+	}
+}
+
 func TestAuth(t *testing.T) {
 	t.Run("Auth for all supported auth methods", func(t *testing.T) {
 		for i, tt := range authTests {