diff --git a/connpool.go b/connpool.go
index b22019c..f774806 100644
--- a/connpool.go
+++ b/connpool.go
@@ -38,8 +38,8 @@ type Pool interface {
 	// no longer usable.
 	Close()
 
-	// Len returns the current number of connections of the pool.
-	Len() int
+	// Size returns the current number of connections of the pool.
+	Size() int
 }
 
 // connPool implements the Pool interface
@@ -174,8 +174,8 @@ func (p *connPool) Close() {
 	}
 }
 
-// Len returns the current number of connections in the connection pool.
-func (p *connPool) Len() int {
+// Size returns the current number of connections in the connection pool.
+func (p *connPool) Size() int {
 	_, conns, _ := p.getConnsAndDialContext()
 	return len(conns)
 }
diff --git a/connpool_test.go b/connpool_test.go
new file mode 100644
index 0000000..589ad68
--- /dev/null
+++ b/connpool_test.go
@@ -0,0 +1,57 @@
+// SPDX-FileCopyrightText: 2022-2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+package mail
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"testing"
+	"time"
+)
+
+func TestNewConnPool(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	serverPort := TestServerPortBase + 10
+	featureSet := "250-AUTH PLAIN\r\n250-8BITMIME\r\n250-DSN\r\n250 SMTPUTF8"
+	go func() {
+		if err := simpleSMTPServer(ctx, featureSet, true, serverPort); err != nil {
+			t.Errorf("failed to start test server: %s", err)
+			return
+		}
+	}()
+	time.Sleep(time.Millisecond * 300)
+
+	pool, err := newConnPool(serverPort)
+	if err != nil {
+		t.Errorf("failed to create connection pool: %s", err)
+	}
+	if pool == nil {
+		t.Errorf("connection pool is nil")
+		return
+	}
+	if pool.Size() != 5 {
+		t.Errorf("expected 5 connections, got %d", pool.Size())
+	}
+	for i := 0; i < 5; i++ {
+		go func() {
+			conn, err := pool.Get()
+			if err != nil {
+				t.Errorf("failed to get connection: %s", err)
+			}
+			if _, err := conn.Write([]byte("EHLO test.localhost.localdomain\r\nQUIT\r\n")); err != nil {
+				t.Errorf("failed to write quit command to first connection: %s", err)
+			}
+		}()
+	}
+}
+
+func newConnPool(port int) (Pool, error) {
+	netDialer := net.Dialer{}
+	return NewConnPool(context.Background(), 5, 30, netDialer.DialContext, "tcp",
+		fmt.Sprintf("127.0.0.1:%d", port))
+}