diff --git a/Bugzilla/Mailer.pm b/Bugzilla/Mailer.pm
index b778e431f..41773fb3e 100644
--- a/Bugzilla/Mailer.pm
+++ b/Bugzilla/Mailer.pm
@@ -98,6 +98,11 @@ sub MessageToMTA {
   $email->header_set('MIME-Version', '1.0')
     if !$email->header('MIME-Version');
 
+  # We ensure there's a Message-ID header set otherwise some mailsystems
+  # treat us as spam.
+  $email->header_set('Message-ID', build_message_id())
+    if !$email->header('Message-ID');
+
   # Encode the headers correctly in quoted-printable
   foreach my $header ($email->header_names) {
     $header = lc $header;
@@ -252,9 +257,9 @@ sub build_thread_marker {
 
   my $sitespec = '@' . Bugzilla->localconfig->urlbase;
   $sitespec =~ s/:\/\//\./;    # Make the protocol look like part of the domain
-  $sitespec =~ s/^([^:\/]+):(\d+)/$1/;    # Remove a port number, to relocate
-  if ($2) {
-    $sitespec = "-$2$sitespec";    # Put the port number back in, before the '@'
+  $sitespec =~ s/\/.*$//;      # Strip path component — / is illegal after @ in Message-ID
+  if ($sitespec =~ s/^([^:\/]+):(\d+)/$1/) {    # Remove port number, to relocate
+    $sitespec = "-$2$sitespec";                  # Put the port number back in, before the '@'
   }
 
   my $threadingmarker = "References: <bug-$bug_id-$user_id$sitespec>";
@@ -270,4 +275,23 @@ sub build_thread_marker {
   return $threadingmarker;
 }
 
+# Builds Message-ID header
+sub build_message_id {
+  my ($user_id) = @_;
+
+  # Don't fall back to current user: this is called from contexts with no logged-in
+  # user (job queue, email_in.pl). The random bits below ensure uniqueness anyway.
+  $user_id //= '';
+
+  my $sitespec = '@' . Bugzilla->localconfig->urlbase;
+  $sitespec =~ s/:\/\//\./;    # Make the protocol look like part of the domain
+  $sitespec =~ s/\/.*$//;      # Strip path component — / is illegal after @ in Message-ID
+  if ($sitespec =~ s/^([^:\/]+):(\d+)/$1/) {    # Remove port number, to relocate
+    $sitespec = "-$2$sitespec";                  # Put the port number back in, before the '@'
+  }
+
+  my $rand_bits  = generate_random_password(10);
+  my $message_id = '<bugzilla-' . ($user_id ne '' ? "$user_id-" : '') . "$rand_bits$sitespec>";
+  return $message_id;
+}
 1;
diff --git a/t/014mailer.t b/t/014mailer.t
new file mode 100644
index 000000000..a3f9dd1d1
--- /dev/null
+++ b/t/014mailer.t
@@ -0,0 +1,113 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# This Source Code Form is "Incompatible With Secondary Licenses", as
+# defined by the Mozilla Public License, v. 2.0.
+
+##################
+#Bugzilla Test 14#
+####Mailer.pm#####
+
+use 5.10.1;
+use strict;
+use warnings;
+
+use lib qw(. lib local/lib/perl5 t);
+use Support::Files;
+use Test::More tests => 18;
+
+BEGIN {
+  use_ok('Bugzilla');
+  use_ok('Bugzilla::Mailer', 'build_thread_marker');
+}
+
+# Inject a stub localconfig into the process cache so the sitespec
+# transformation functions can be tested without a real installation.
+{
+
+  package Bugzilla::Test::FakeLocalconfig;
+
+  sub new { my ($class, $urlbase) = @_; bless {urlbase => $urlbase}, $class }
+  sub urlbase { $_[0]->{urlbase} }
+}
+
+sub set_urlbase {
+  my ($urlbase) = @_;
+  Bugzilla->process_cache->{localconfig}
+    = Bugzilla::Test::FakeLocalconfig->new($urlbase);
+}
+
+# ---- build_message_id: sitespec transformation ----
+#
+# The central invariant: whatever urlbase looks like, the resulting
+# Message-ID must be a valid <local-part@domain> with no '/' characters
+# after the '@'.
+
+my $mid;
+
+# Plain http, trailing slash only — baseline case.
+set_urlbase('http://bugs.example.org/');
+$mid = Bugzilla::Mailer::build_message_id();
+like($mid, qr/^<bugzilla-[A-Za-z0-9]+\@http\.bugs\.example\.org>$/,
+  'simple http urlbase: correct format and sitespec');
+unlike($mid, qr/\@[^>]*\//, 'simple http urlbase: no slash after @');
+
+# Path component after the hostname — the key regression this patch fixes.
+set_urlbase('https://bugs.example.org/bugzilla/');
+$mid = Bugzilla::Mailer::build_message_id();
+like($mid, qr/^<bugzilla-[A-Za-z0-9]+\@https\.bugs\.example\.org>$/,
+  'https with path: path component stripped from sitespec');
+unlike($mid, qr/\@[^>]*\//, 'https with path: no slash after @');
+
+# Non-standard port — port must move before the '@'.
+set_urlbase('https://bugs.example.org:8080/');
+$mid = Bugzilla::Mailer::build_message_id();
+like($mid, qr/^<bugzilla-[A-Za-z0-9]+-8080\@https\.bugs\.example\.org>$/,
+  'https with port: port relocated before @');
+unlike($mid, qr/\@[^>]*\//, 'https with port: no slash after @');
+
+# Port and path together.
+set_urlbase('https://bugs.example.org:8080/bugzilla/');
+$mid = Bugzilla::Mailer::build_message_id();
+like($mid, qr/^<bugzilla-[A-Za-z0-9]+-8080\@https\.bugs\.example\.org>$/,
+  'https with port and path: path stripped, port relocated');
+unlike($mid, qr/\@[^>]*\//, 'https with port and path: no slash after @');
+
+# ---- build_message_id: user_id handling ----
+
+set_urlbase('https://bugs.example.org/');
+
+my $mid_with_user = Bugzilla::Mailer::build_message_id(42);
+like($mid_with_user, qr/^<bugzilla-42-[A-Za-z0-9]+\@/,
+  'with user_id: user_id present in local-part');
+unlike($mid_with_user, qr/bugzilla--/,
+  'with user_id: no double-dash');
+
+my $mid_no_user = Bugzilla::Mailer::build_message_id();
+like($mid_no_user, qr/^<bugzilla-[A-Za-z0-9]+\@/,
+  'without user_id: clean local-part (no double-dash)');
+unlike($mid_no_user, qr/bugzilla--/,
+  'without user_id: no double-dash');
+
+# ---- build_thread_marker: same sitespec logic ----
+
+my $marker;
+
+# Path component must be stripped here too.
+set_urlbase('https://bugs.example.org/bugzilla/');
+$marker = build_thread_marker(99, 7, 1);    # new bug
+like($marker, qr/Message-ID: <bug-99-7\@https\.bugs\.example\.org>/,
+  'new thread with path: path stripped from sitespec');
+unlike($marker, qr/\@[^>]*\//, 'new thread with path: no slash after @');
+
+# Port relocation for non-standard port.
+set_urlbase('https://bugs.example.org:8080/');
+$marker = build_thread_marker(99, 7, 1);    # new bug
+like($marker, qr/Message-ID: <bug-99-7-8080\@https\.bugs\.example\.org>/,
+  'new thread with port: port relocated before @');
+
+# Reply includes In-Reply-To pointing at the root message.
+$marker = build_thread_marker(99, 7, 0);    # reply
+like($marker, qr/In-Reply-To: <bug-99-7-8080\@https\.bugs\.example\.org>/,
+  'reply thread: In-Reply-To uses correct sitespec');