diff --git a/extensions/MozChangeField/Extension.pm b/extensions/MozChangeField/Extension.pm
index 6e2dbffdae..860b0fdad7 100644
--- a/extensions/MozChangeField/Extension.pm
+++ b/extensions/MozChangeField/Extension.pm
@@ -34,18 +34,22 @@ my @pre_instances = (
 
 use Bugzilla::Extension::MozChangeField::Post::SeverityS1PriorityP1;
 use Bugzilla::Extension::MozChangeField::Post::ClearTrackingPriorityS1;
+
 #use Bugzilla::Extension::MozChangeField::Post::CommentOnSeverity;
 use Bugzilla::Extension::MozChangeField::Post::SetTrackingSeverityS1;
 use Bugzilla::Extension::MozChangeField::Post::TypePriSevEditbugs;
 use Bugzilla::Extension::MozChangeField::Post::RegressedByTypeKeyword;
+use Bugzilla::Extension::MozChangeField::Post::CheckinNeededTbMilestone;
 
 my @post_instances = (
   Bugzilla::Extension::MozChangeField::Post::SeverityS1PriorityP1->new,
   Bugzilla::Extension::MozChangeField::Post::ClearTrackingPriorityS1->new,
+
   #Bugzilla::Extension::MozChangeField::Post::CommentOnSeverity->new,
   Bugzilla::Extension::MozChangeField::Post::SetTrackingSeverityS1->new,
   Bugzilla::Extension::MozChangeField::Post::TypePriSevEditbugs->new,
   Bugzilla::Extension::MozChangeField::Post::RegressedByTypeKeyword->new,
+  Bugzilla::Extension::MozChangeField::Post::CheckinNeededTbMilestone->new,
 );
 
 our $VERSION = '0.1';
diff --git a/extensions/MozChangeField/lib/Post/CheckinNeededTbMilestone.pm b/extensions/MozChangeField/lib/Post/CheckinNeededTbMilestone.pm
new file mode 100644
index 0000000000..d52db2b615
--- /dev/null
+++ b/extensions/MozChangeField/lib/Post/CheckinNeededTbMilestone.pm
@@ -0,0 +1,53 @@
+# 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.
+
+package Bugzilla::Extension::MozChangeField::Post::CheckinNeededTbMilestone;
+
+use 5.10.1;
+use Moo;
+
+use Bugzilla::Error;
+
+use constant PRODUCTS =>
+  {map { $_ => 1 } ('Thunderbird', 'MailNews Core', 'Calendar')};
+
+use constant KEYWORD => 'checkin-needed-tb';
+
+sub _check_milestone {
+  my ($bug) = @_;
+  my $product = $bug->product_obj;
+
+  return if !PRODUCTS->{$product->name};
+  return if !$bug->has_keyword(KEYWORD);
+
+  # The product's default milestone (normally '---') means 'not set'.
+  return if $bug->target_milestone ne $product->default_milestone;
+
+  ThrowUserError('mozchangefield_checkin_needed_tb_milestone',
+    {keyword => KEYWORD});
+}
+
+sub evaluate_create {
+  my ($self, $args) = @_;
+  _check_milestone($args->{bug});
+}
+
+sub evaluate_change {
+  my ($self, $args) = @_;
+  my $changes = $args->{changes};
+
+  # Only enforce on a save that touches one of three fields, so bugs
+  # already in this state don't block unrelated edits.
+  return
+       if !exists $changes->{keywords}
+    && !exists $changes->{target_milestone}
+    && !exists $changes->{product};
+
+  _check_milestone($args->{bug});
+}
+
+1;
diff --git a/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl b/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl
index 62ca30ec20..777ecc516f 100644
--- a/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl
+++ b/extensions/MozChangeField/template/en/default/hook/global/user-error-errors.html.tmpl
@@ -14,4 +14,9 @@
   [% title = "Invalid Change" %]
   You need "editbugs" permissions to alter the '[% field FILTER html %]' field.
 
+[% ELSIF error == "mozchangefield_checkin_needed_tb_milestone" %]
+  [% title = "Target Milestone Required" %]
+  The <b>[% keyword FILTER html %]</b> keyword requires a
+  <b>Target Milestone</b> to be set.
+
 [% END %]