diff --git a/spec/sparoid_spec.cr b/spec/sparoid_spec.cr
index 9030ddf..95f6614 100644
--- a/spec/sparoid_spec.cr
+++ b/spec/sparoid_spec.cr
@@ -142,3 +142,43 @@ describe Sparoid::Server do
     s.try &.close
   end
 end
+
+describe Sparoid::Client do
+  describe ".process_send_results" do
+    it "raises SendError when every send failed" do
+      ip1 = Socket::IPAddress.new("1.1.1.1", 1)
+      ip2 = Socket::IPAddress.new("2.2.2.2", 2)
+      results = [
+        {ip1, Exception.new("no route").as(Exception?)},
+        {ip2, Exception.new("network down").as(Exception?)},
+      ]
+      ex = expect_raises(Sparoid::Client::SendError) do
+        Sparoid::Client.process_send_results("example.com", results)
+      end
+      ex.message.to_s.should contain("example.com")
+      ex.message.to_s.should contain("1.1.1.1:1: no route")
+      ex.message.to_s.should contain("2.2.2.2:2: network down")
+    end
+
+    it "returns partial-failure errors when at least one send succeeded" do
+      ip1 = Socket::IPAddress.new("1.1.1.1", 1)
+      ip2 = Socket::IPAddress.new("2.2.2.2", 2)
+      err = Exception.new("oops")
+      results = [
+        {ip1, nil.as(Exception?)},
+        {ip2, err.as(Exception?)},
+      ]
+      Sparoid::Client.process_send_results("example.com", results).should eq [{ip2, err}]
+    end
+
+    it "returns nothing when all sends succeeded" do
+      ip = Socket::IPAddress.new("1.1.1.1", 1)
+      results = [{ip, nil.as(Exception?)}]
+      Sparoid::Client.process_send_results("example.com", results).should be_empty
+    end
+
+    it "returns nothing when there were no addresses" do
+      Sparoid::Client.process_send_results("example.com", [] of {Socket::IPAddress, Exception?}).should be_empty
+    end
+  end
+end
diff --git a/src/client-cli.cr b/src/client-cli.cr
index 338ebb6..17b45cf 100644
--- a/src/client-cli.cr
+++ b/src/client-cli.cr
@@ -1,7 +1,11 @@
+require "log"
 require "option_parser"
 require "./client"
 require "./version"
 
+# Route logs to STDERR (in :connect mode STDOUT is the unix-domain socket used for FD passing).
+Log.setup_from_env(default_level: :info, backend: Log::IOBackend.new(STDERR))
+
 subcommand = :none
 host = "0.0.0.0"
 port = 8484
diff --git a/src/client.cr b/src/client.cr
index 6147c9d..e525745 100644
--- a/src/client.cr
+++ b/src/client.cr
@@ -1,3 +1,4 @@
+require "log"
 require "socket"
 require "random/secure"
 require "openssl/cipher"
@@ -11,6 +12,10 @@ require "wait_group"
 
 module Sparoid
   class Client
+    Log = ::Log.for(self)
+
+    class SendError < Exception; end
+
     def self.new(config_path = "~/.sparoid.ini")
       key = ENV.fetch("SPAROID_KEY", "")
       hmac_key = ENV.fetch("SPAROID_HMAC_KEY", "")
@@ -70,25 +75,43 @@ module Sparoid
       exit 1 # only if all connects fails
     end
 
-    # Send to all resolved IPs for the hostname
+    # Send to all resolved IPs for the hostname.
+    # Per-address failures are logged as warnings if at least one address succeeded.
+    # If every address fails, raises SendError.
     private def self.udp_send(host, port, key : String, hmac_key : String, public_ip : String? = nil) : Array(String)
       host_addresses = Socket::Addrinfo.udp(host, port)
-      host_addresses.each do |addrinfo|
+      results = host_addresses.map do |addrinfo|
         packages = generate_messages(addrinfo.ip_address, public_ip).map { |message| generate_package(key, hmac_key, message) }
         socket = UDPSocket.new(addrinfo.family)
+        error = nil.as(Exception?)
         begin
           packages.each do |data|
             socket.send data, to: addrinfo.ip_address
           end
         rescue ex
-          STDERR << "Sparoid error sending " << ex.inspect << "\n"
+          error = ex
         ensure
           socket.close
         end
+        {addrinfo.ip_address, error}
+      end
+      process_send_results(host, results).each do |ip, ex|
+        Log.warn { "skip #{host} (#{ip}): #{ex.message}" }
       end
       host_addresses.map &.ip_address.address
     end
 
+    # Decide whether per-address send failures are partial (warn) or total (raise).
+    # Returns the per-address errors to warn about. Raises SendError when every send failed.
+    def self.process_send_results(host : String, results : Array({Socket::IPAddress, Exception?})) : Array({Socket::IPAddress, Exception})
+      errors = results.compact_map { |ip, err| err.try { |e| {ip, e} } }
+      if !results.empty? && errors.size == results.size
+        details = errors.map { |ip, ex| "#{ip}: #{ex.message}" }.join("; ")
+        raise SendError.new("failed to send to any address for #{host}: #{details}")
+      end
+      errors
+    end
+
     private def self.encrypt(key, hmac_key, data) : Bytes
       cipher = OpenSSL::Cipher.new("aes-256-cbc")
       cipher.encrypt