diff --git a/src/Protocol/Imap/Negotiation/AuthNegotiation.php b/src/Protocol/Imap/Negotiation/AuthNegotiation.php
index 9604f538..52134991 100644
--- a/src/Protocol/Imap/Negotiation/AuthNegotiation.php
+++ b/src/Protocol/Imap/Negotiation/AuthNegotiation.php
@@ -3,6 +3,7 @@
 
 namespace Genkgo\Mail\Protocol\Imap\Negotiation;
 
+use Genkgo\Mail\Exception\AssertionFailedException;
 use Genkgo\Mail\Exception\ImapAuthenticationException;
 use Genkgo\Mail\Protocol\Imap\Client;
 use Genkgo\Mail\Protocol\Imap\NegotiationInterface;
@@ -88,7 +89,7 @@ public function negotiate(Client $client): void
                     ->first()
                     ->assertContinuation();
 
-                $client
+                $taggedResponse = $client
                     ->emit(
                         new AuthPlainCredentialsRequest(
                             $tag,
@@ -97,16 +98,29 @@ public function negotiate(Client $client): void
                         )
                     )
                     ->last()
-                    ->assertCompletion(CompletionResult::ok())
                     ->assertTagged();
+
+                try {
+                    $taggedResponse->assertCompletion(CompletionResult::ok());
+                } catch (AssertionFailedException $e) {
+                    throw new ImapAuthenticationException(
+                        'Failed to authenticate: ' . $taggedResponse->getBody()
+                    );
+                }
                 break;
             case Client::AUTH_LOGIN:
-                $tag = $client->newTag();
-                $client
-                    ->emit(new LoginCommand($tag, $this->username, $this->password))
+                $taggedResponse = $client
+                    ->emit(new LoginCommand($client->newTag(), $this->username, $this->password))
                     ->last()
-                    ->assertTagged()
-                    ->assertCompletion(CompletionResult::ok());
+                    ->assertTagged();
+
+                try {
+                    $taggedResponse->assertCompletion(CompletionResult::ok());
+                } catch (AssertionFailedException $e) {
+                    throw new ImapAuthenticationException(
+                        'Failed to authenticate: ' . $taggedResponse->getBody()
+                    );
+                }
                 break;
         }
     }
diff --git a/test/Unit/Protocol/Imap/Negotiation/AuthNegotiationTest.php b/test/Unit/Protocol/Imap/Negotiation/AuthNegotiationTest.php
index c392a159..e2cf83b6 100644
--- a/test/Unit/Protocol/Imap/Negotiation/AuthNegotiationTest.php
+++ b/test/Unit/Protocol/Imap/Negotiation/AuthNegotiationTest.php
@@ -172,4 +172,92 @@ public function it_uses_capability_when_method_is_auto_and_not_advertised_and_lo
         $negotiation = new AuthNegotiation(Client::AUTH_AUTO, 'username', 'password');
         $negotiation->negotiate($client);
     }
+
+    /**
+     * @test
+     */
+    public function it_throws_auth_exception_when_plain_auth_fails(): void
+    {
+        $this->expectException(ImapAuthenticationException::class);
+        $this->expectExceptionMessage('Failed to authenticate: NO [AUTHENTICATIONFAILED] Authentication failed.');
+
+        $connection = $this->createMock(ConnectionInterface::class);
+        $connection
+            ->expects($this->at(0))
+            ->method('addListener');
+
+        $connection
+            ->expects($this->at(1))
+            ->method('send')
+            ->with("TAG1 AUTHENTICATE PLAIN\r\n");
+
+        $connection
+            ->expects($this->at(2))
+            ->method('receive')
+            ->willReturn('+ Send password');
+
+        $connection
+            ->expects($this->at(3))
+            ->method('send')
+            ->with("AHVzZXJuYW1lAHBhc3N3b3Jk\r\n");
+
+        $connection
+            ->expects($this->at(4))
+            ->method('receive')
+            ->willReturn('TAG1 NO [AUTHENTICATIONFAILED] Authentication failed.');
+
+        $client = new Client($connection, new GeneratorTagFactory(), []);
+
+        $negotiation = new AuthNegotiation(Client::AUTH_PLAIN, 'username', 'password');
+        $negotiation->negotiate($client);
+    }
+
+    /**
+     * @test
+     */
+    public function it_throws_auth_exception_when_login_fails(): void
+    {
+        $this->expectException(ImapAuthenticationException::class);
+        $this->expectExceptionMessage('Failed to authenticate: NO [AUTHENTICATIONFAILED] Authentication failed.');
+
+        $connection = $this->createMock(ConnectionInterface::class);
+        $connection
+            ->expects($this->at(0))
+            ->method('addListener');
+
+        $connection
+            ->expects($this->at(1))
+            ->method('send')
+            ->with("TAG1 CAPABILITY\r\n");
+
+        $connection
+            ->expects($this->at(2))
+            ->method('receive')
+            ->willReturn('* CAPABILITY IMAP4rev1 STARTTLS');
+
+        $connection
+            ->expects($this->at(3))
+            ->method('receive')
+            ->willReturn('TAG1 OK');
+
+        $connection
+            ->expects($this->at(4))
+            ->method('send')
+            ->with("TAG2 LOGIN username password\r\n");
+
+        $connection
+            ->expects($this->at(5))
+            ->method('receive')
+            ->willReturn('TAG2 NO [AUTHENTICATIONFAILED] Authentication failed.');
+
+        $client = new Client($connection, new GeneratorTagFactory(), []);
+
+        $negotiation = new AuthNegotiation(Client::AUTH_AUTO, 'username', 'password');
+        $negotiation->negotiate($client);
+
+        $client = new Client($connection, new GeneratorTagFactory(), []);
+
+        $negotiation = new AuthNegotiation(Client::AUTH_PLAIN, 'username', 'password');
+        $negotiation->negotiate($client);
+    }
 }