diff --git a/lib/mix/tasks/sentry.send_test_event.ex b/lib/mix/tasks/sentry.send_test_event.ex
index 7a740733..dcf07d48 100644
--- a/lib/mix/tasks/sentry.send_test_event.ex
+++ b/lib/mix/tasks/sentry.send_test_event.ex
@@ -26,6 +26,8 @@ defmodule Mix.Tasks.Sentry.SendTestEvent do
     type: :string
   ]
 
+  @requirements ["app.start"]
+
   @impl true
   def run(args) when is_list(args) do
     {opts, args} = OptionParser.parse!(args, strict: @switches)
diff --git a/lib/sentry/config.ex b/lib/sentry/config.ex
index a2407512..d6c50e13 100644
--- a/lib/sentry/config.ex
+++ b/lib/sentry/config.ex
@@ -322,6 +322,14 @@ defmodule Sentry.Config do
       connections to keep in the pool. Only applied if `:client` is set to
       `Sentry.HackneyClient`.
       """
+    ],
+    req_opts: [
+      type: :keyword_list,
+      default: [],
+      doc: """
+      Options to be passed to `Req`. Only
+      applied if `:client` is set to `Sentry.ReqClient`.
+      """
     ]
   ]
 
@@ -559,6 +567,9 @@ defmodule Sentry.Config do
   @spec hackney_opts() :: keyword()
   def hackney_opts, do: fetch!(:hackney_opts)
 
+  @spec req_opts() :: keyword()
+  def req_opts, do: fetch!(:req_opts)
+
   @spec before_send() :: (Sentry.Event.t() -> Sentry.Event.t()) | {module(), atom()} | nil
   def before_send, do: get(:before_send)
 
diff --git a/lib/sentry/req_client.ex b/lib/sentry/req_client.ex
new file mode 100644
index 00000000..abd81e48
--- /dev/null
+++ b/lib/sentry/req_client.ex
@@ -0,0 +1,32 @@
+defmodule Sentry.ReqClient do
+  @behaviour Sentry.HTTPClient
+
+  @moduledoc """
+  The built-in HTTP client.
+
+  This client implements the `Sentry.HTTPClient` behaviour.
+
+  It's based on the [Req](https://github.com/wojtekmach/req) HTTP client,
+  which is an *optional dependency* of this library. If you wish to use another
+  HTTP client, you'll have to implement your own `Sentry.HTTPClient`. See the
+  documentation for `Sentry.HTTPClient` for more information.
+  """
+
+  @impl true
+  def post(url, headers, body) do
+    opts =
+      Sentry.Config.req_opts()
+      |> Keyword.put(:decode_body, false)
+      |> Keyword.put(:url, url)
+      |> Keyword.put(:body, body)
+
+    req =
+      Req.new(opts)
+      |> Req.merge(headers: headers)
+
+    case Req.post(req) do
+      {:ok, %{status: status, headers: headers, body: body}} = result -> {:ok, status, headers, body}
+      {:error, _reason} = error -> error
+    end
+  end
+end
diff --git a/lib/sentry/transport.ex b/lib/sentry/transport.ex
index a353b2b8..d9fc27e3 100644
--- a/lib/sentry/transport.ex
+++ b/lib/sentry/transport.ex
@@ -125,7 +125,7 @@ defmodule Sentry.Transport do
   defp client_post_and_validate_return_value(client, endpoint, headers, body) do
     case client.post(endpoint, headers, body) do
       {:ok, status, resp_headers, resp_body}
-      when is_integer(status) and status in 200..599 and is_list(resp_headers) and
+      when is_integer(status) and status in 200..599 and
              is_binary(resp_body) ->
         {:ok, status, resp_headers, resp_body}
 
@@ -152,8 +152,8 @@ defmodule Sentry.Transport do
       |> Enum.map_join(", ", fn {name, value} -> "#{name}=#{value}" end)
 
     auth_headers = [
-      {"User-Agent", @sentry_client},
-      {"X-Sentry-Auth", "Sentry " <> auth_query}
+      {"user-agent", @sentry_client},
+      {"x-sentry-auth", "Sentry " <> auth_query}
     ]
 
     {dsn.endpoint_uri, auth_headers}
diff --git a/mix.exs b/mix.exs
index acfb35fa..6e75c92d 100644
--- a/mix.exs
+++ b/mix.exs
@@ -51,7 +51,7 @@ defmodule Sentry.Mixfile do
           "Plug and Phoenix": [Sentry.PlugCapture, Sentry.PlugContext, Sentry.LiveViewHook],
           Loggers: [Sentry.LoggerBackend, Sentry.LoggerHandler],
           "Data Structures": [Sentry.Attachment, Sentry.CheckIn, Sentry.ClientReport],
-          HTTP: [Sentry.HTTPClient, Sentry.HackneyClient],
+          HTTP: [Sentry.HTTPClient, Sentry.HackneyClient, Sentry.ReqClient],
           Interfaces: [~r/^Sentry\.Interfaces/],
           Testing: [Sentry.Test]
         ],