diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9634eec..01219b3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,18 @@ you were catching the `TypeError` from any of the getters above as a workaround,
   always permitted; dropping 8.1 is the only real constraint change.
 
 ### Changed
+- **BREAKING** — **transport failures are now `PostmarkTransportException`, not Guzzle exceptions.**
+  `processRestRequest()` previously documented `@throws \GuzzleHttp\Exception\GuzzleException`,
+  making Guzzle's hierarchy part of this SDK's public contract. Because the SDK disables
+  `http_errors` and maps responses itself, the transport family was the *only* Guzzle family that
+  could reach your code — so a Guzzle upgrade silently changed what your `catch` blocks matched.
+  Anything that never reached the API (DNS, connect, TLS, timeout, socket) is now
+  `Postmark\Models\PostmarkTransportException`, which extends `PostmarkException`, so existing
+  `catch (PostmarkException $e)` blocks keep working. The original is preserved on `getPrevious()`.
+
+  Replace `catch (\GuzzleHttp\Exception\ConnectException $e)` with
+  `catch (\Postmark\Models\PostmarkTransportException $e)` and use `$e->isTimeout()` /
+  `$e->isConnectionFailure()` — the SDK normalises across Guzzle majors so you don't have to.
 - **BREAKING** — `PostmarkAttachment::fromRawData()`, `::fromBase64EncodedData()` and `::fromFile()`
   now declare `string` for their first two parameters and a `PostmarkAttachment` return type.
   Passing `null`, an array, or a non-Stringable object now raises a `TypeError`; previously it
diff --git a/src/Postmark/Models/PostmarkTransportException.php b/src/Postmark/Models/PostmarkTransportException.php
new file mode 100644
index 0000000..4ce5961
--- /dev/null
+++ b/src/Postmark/Models/PostmarkTransportException.php
@@ -0,0 +1,74 @@
+<?php
+
+namespace Postmark\Models;
+
+use Throwable;
+
+/**
+ * Thrown when the request never produced an HTTP response.
+ *
+ * DNS failure, connection refused, TLS handshake failure, timeout, socket error —
+ * anything where the API was not reached. A response that arrives and carries an
+ * error is a plain {@see PostmarkException} instead.
+ *
+ * This exists so callers never have to catch their HTTP client's exception types.
+ * The SDK previously documented `@throws \GuzzleHttp\Exception\GuzzleException`,
+ * which made Guzzle's hierarchy part of the public contract — and Guzzle 8
+ * reclassified exactly that family, so `catch (ConnectException)` around a send
+ * silently stopped matching a timeout.
+ *
+ * The class is deliberately free of any Guzzle reference: the client normalises
+ * the failure to one of the KIND_* values before constructing this, so all
+ * version-specific knowledge lives in one place.
+ */
+class PostmarkTransportException extends PostmarkException
+{
+    /** No response within the configured timeout. */
+    public const KIND_TIMEOUT = 'timeout';
+
+    /** Connection could not be established, or was lost before a response. */
+    public const KIND_CONNECTION = 'connection';
+
+    /** Reached the transport layer, but the cause could not be determined. */
+    public const KIND_UNKNOWN = 'unknown';
+
+    private string $kind;
+
+    public function __construct(string $message, ?Throwable $previous = null, string $kind = self::KIND_UNKNOWN)
+    {
+        parent::__construct($message, 0, $previous);
+
+        // PostmarkException redeclares $message as a public property, so the value
+        // handed to the parent constructor is not what getMessage() reports unless
+        // it is assigned here too.
+        $this->message = $message;
+        $this->kind = $kind;
+    }
+
+    /**
+     * The request was not answered within the configured timeout.
+     *
+     * Retry is at-least-once, not safe-to-repeat: a timeout means no response was
+     * seen, which does not prove the message was rejected.
+     */
+    public function isTimeout(): bool
+    {
+        return self::KIND_TIMEOUT === $this->kind;
+    }
+
+    /**
+     * The connection failed before a response could be produced.
+     *
+     * Safe to retry — the request did not complete.
+     */
+    public function isConnectionFailure(): bool
+    {
+        return self::KIND_CONNECTION === $this->kind;
+    }
+
+    /** One of the KIND_* constants. */
+    public function getKind(): string
+    {
+        return $this->kind;
+    }
+}
diff --git a/src/Postmark/PostmarkClientBase.php b/src/Postmark/PostmarkClientBase.php
index 9ed5e52..e981f36 100644
--- a/src/Postmark/PostmarkClientBase.php
+++ b/src/Postmark/PostmarkClientBase.php
@@ -9,8 +9,10 @@
 namespace Postmark;
 
 use GuzzleHttp\Client;
+use GuzzleHttp\Exception\GuzzleException;
 use GuzzleHttp\RequestOptions;
 use Postmark\Models\PostmarkException;
+use Postmark\Models\PostmarkTransportException;
 
 /**
  * This is the core class that interacts with the Postmark API. All clients should
@@ -93,6 +95,55 @@ protected function getClient()
         return $this->client;
     }
 
+    /**
+     * Normalise a Guzzle transport failure to a PostmarkTransportException kind.
+     *
+     * All Guzzle-version knowledge is confined here. The two majors express the
+     * same failures differently:
+     *   - Guzzle 7 raises ConnectException for connect AND timeout, and carries a
+     *     cURL errno on the handler context, which is what separates them.
+     *   - Guzzle 8 re-parents ConnectException under NetworkException, DROPS
+     *     getHandlerContext(), and adds precise subclasses instead
+     *     (ConnectTimeoutException, NetworkTimeoutException, ResponseTimeoutException).
+     * Matching on the short class name keeps this working under both without
+     * referencing classes that only exist in one.
+     */
+    private static function classifyTransportFailure(GuzzleException $e): string
+    {
+        $shortName = substr(strrchr('\\' . get_class($e), '\\') ?: '', 1);
+
+        // Guzzle 8: the class already says which it is.
+        if (in_array($shortName, ['ConnectTimeoutException', 'NetworkTimeoutException', 'ResponseTimeoutException'], true)) {
+            return PostmarkTransportException::KIND_TIMEOUT;
+        }
+
+        // Guzzle 7: disambiguate with the cURL errno when the handler supplied one.
+        if (method_exists($e, 'getHandlerContext')) {
+            $errno = $e->getHandlerContext()['errno'] ?? null;
+
+            if (28 === $errno) {
+                return PostmarkTransportException::KIND_TIMEOUT;
+            }
+
+            // Resolve, connect, TLS, empty reply, send/recv.
+            if (in_array($errno, [5, 6, 7, 35, 52, 55, 56], true)) {
+                return PostmarkTransportException::KIND_CONNECTION;
+            }
+        }
+
+        $message = strtolower($e->getMessage());
+
+        if (str_contains($message, 'timed out') || str_contains($message, 'timeout')) {
+            return PostmarkTransportException::KIND_TIMEOUT;
+        }
+
+        if (in_array($shortName, ['ConnectException', 'NetworkException'], true)) {
+            return PostmarkTransportException::KIND_CONNECTION;
+        }
+
+        return PostmarkTransportException::KIND_UNKNOWN;
+    }
+
     /**
      * The base request method for all API access.
      *
@@ -102,8 +153,8 @@ protected function getClient()
      *
      * @return mixed
      *
-     * @throws PostmarkException
-     * @throws \GuzzleHttp\Exception\GuzzleException
+     * @throws PostmarkException          if the API returns an error response
+     * @throws PostmarkTransportException if the API could not be reached at all
      */
     protected function processRestRequest($method = null, $path = null, array $body = []): mixed
     {
@@ -142,7 +193,20 @@ protected function processRestRequest($method = null, $path = null, array $body
             }
         }
 
-        $response = $client->request($method, self::$BASE_URL . $path, $options);
+        try {
+            $response = $client->request($method, self::$BASE_URL . $path, $options);
+        } catch (GuzzleException $e) {
+            // The HTTP client's exception hierarchy is not part of this SDK's contract.
+            // Guzzle 8 reclassified the transport family — a plain timeout stopped being
+            // a ConnectException — and with HTTP_ERRORS disabled that family is the only
+            // one that can reach a caller, so leaking it made every consumer's catch
+            // block version-dependent.
+            throw new PostmarkTransportException(
+                sprintf('Could not reach the Postmark API: %s', $e->getMessage()),
+                $e,
+                self::classifyTransportFailure($e)
+            );
+        }
 
         switch ($response->getStatusCode()) {
             case 200:
diff --git a/tests/TransportExceptionTest.php b/tests/TransportExceptionTest.php
new file mode 100644
index 0000000..e64508b
--- /dev/null
+++ b/tests/TransportExceptionTest.php
@@ -0,0 +1,141 @@
+<?php
+
+namespace Postmark\Tests;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+use GuzzleHttp\Client;
+use GuzzleHttp\Exception\ConnectException;
+use GuzzleHttp\Exception\GuzzleException;
+use GuzzleHttp\Handler\MockHandler;
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Request;
+use PHPUnit\Framework\TestCase;
+use Postmark\Models\PostmarkException;
+use Postmark\Models\PostmarkTransportException;
+use Postmark\PostmarkClient;
+
+/**
+ * The SDK must not leak its HTTP client's exception hierarchy.
+ *
+ * Guzzle 8 reclassified transport exceptions — a plain timeout stopped being a
+ * ConnectException — and because the SDK disables http_errors, that family is
+ * the only one that can reach a caller. These run identically under Guzzle 7
+ * and 8; the CI matrix exercises both. No credentials required.
+ */
+class TransportExceptionTest extends TestCase
+{
+    public function testTransportFailureIsWrappedInAPostmarkType(): void
+    {
+        $client = $this->clientThatFailsWith(
+            new ConnectException('cURL error 7: Failed to connect', new Request('POST', 'test'))
+        );
+
+        try {
+            $client->sendEmail('a@example.com', 'b@example.com', 'Subject', null, 'Body');
+            $this->fail('Expected a PostmarkTransportException.');
+        } catch (PostmarkTransportException $e) {
+            $this->assertStringContainsString('Could not reach the Postmark API', $e->getMessage());
+            $this->assertInstanceOf(ConnectException::class, $e->getPrevious());
+        }
+    }
+
+    /** Existing `catch (PostmarkException)` blocks must keep working. */
+    public function testItIsCatchableAsAPostmarkException(): void
+    {
+        $client = $this->clientThatFailsWith(
+            new ConnectException('cURL error 7', new Request('POST', 'test'))
+        );
+
+        $this->expectException(PostmarkException::class);
+        $client->sendEmail('a@example.com', 'b@example.com', 'Subject', null, 'Body');
+    }
+
+    /**
+     * The point of the change: a caller can classify the failure without
+     * importing anything from GuzzleHttp, and gets the same answer on either
+     * major even though the underlying class differs.
+     */
+    public function testCallersCanClassifyWithoutTouchingGuzzle(): void
+    {
+        $timeout = $this->captureFrom($this->connectFailure('cURL error 28: Operation timed out', 28));
+
+        $this->assertTrue($timeout->isTimeout());
+        $this->assertFalse($timeout->isConnectionFailure());
+
+        $refused = $this->captureFrom($this->connectFailure('cURL error 7: Failed to connect', 7));
+
+        $this->assertTrue($refused->isConnectionFailure());
+        $this->assertFalse($refused->isTimeout());
+    }
+
+    /** Handlers that report no errno still classify, via the message. */
+    public function testClassificationFallsBackToTheMessageWithoutAnErrno(): void
+    {
+        $timeout = $this->captureFrom(
+            new ConnectException('cURL error 28: Operation timed out', new Request('POST', 'test'))
+        );
+
+        $this->assertTrue($timeout->isTimeout());
+    }
+
+    /** A response that arrives carrying an error is NOT a transport failure. */
+    public function testApiErrorsAreStillPlainPostmarkExceptions(): void
+    {
+        $mock = new MockHandler([new \GuzzleHttp\Psr7\Response(401, [], '{}')]);
+        $guzzle = new Client(['handler' => HandlerStack::create($mock)]);
+        $client = new PostmarkClient('test-token');
+        $client->setClient($guzzle);
+
+        try {
+            $client->sendEmail('a@example.com', 'b@example.com', 'Subject', null, 'Body');
+            $this->fail('Expected a PostmarkException.');
+        } catch (PostmarkTransportException $e) {
+            $this->fail('A 401 is a response, not a transport failure.');
+        } catch (PostmarkException $e) {
+            $this->assertSame(401, $e->getHttpStatusCode());
+        }
+    }
+
+    /**
+     * Guzzle 7's ConnectException accepts a handler context (which is where the
+     * cURL errno lives); Guzzle 8 removed that parameter and uses precise
+     * subclasses instead. Build whichever the installed major supports.
+     */
+    private function connectFailure(string $message, int $errno): GuzzleException
+    {
+        $class = new \ReflectionClass(ConnectException::class);
+        $arguments = [$message, new Request('POST', 'test')];
+
+        // Built reflectively rather than with a literal 4-argument call: PHPStan
+        // resolves the constructor against whichever major is installed and would
+        // reject the wider form under Guzzle 8, even guarded.
+        if ($class->getConstructor()->getNumberOfParameters() >= 4) {
+            $arguments[] = null;
+            $arguments[] = ['errno' => $errno];
+        }
+
+        return $class->newInstanceArgs($arguments);
+    }
+
+    private function captureFrom(GuzzleException $failure): PostmarkTransportException
+    {
+        try {
+            $this->clientThatFailsWith($failure)
+                ->sendEmail('a@example.com', 'b@example.com', 'Subject', null, 'Body');
+        } catch (PostmarkTransportException $e) {
+            return $e;
+        }
+
+        $this->fail('Expected a PostmarkTransportException.');
+    }
+
+    private function clientThatFailsWith(GuzzleException $failure): PostmarkClient
+    {
+        $guzzle = new Client(['handler' => HandlerStack::create(new MockHandler([$failure]))]);
+        $client = new PostmarkClient('test-token');
+        $client->setClient($guzzle);
+
+        return $client;
+    }
+}