Add signal global_email_filter

src/pretix/base/services/mail.py

@@ -29,7 +29,7 @@ from pretix.base.models import (
 from pretix.base.services.invoices import invoice_pdf_task
 from pretix.base.services.tasks import TransactionAwareTask
 from pretix.base.services.tickets import get_tickets_for_order
-from pretix.base.signals import email_filter
+from pretix.base.signals import email_filter, global_email_filter
 from pretix.celery_app import app
 from pretix.multidomain.urlreverse import build_absolute_uri
 
@@ -305,6 +305,8 @@ def mail_send_task(self, *args, to: List[str], subject: str, body: str, html: st
 
             email = email_filter.send_chained(event, 'message', message=email, order=order, user=user)
 
+        email = global_email_filter.send_chained(None, 'message', message=email, user=user)
+
         try:
             backend.send_messages([email])
         except smtplib.SMTPResponseException as e:

src/pretix/base/signals.py

@@ -514,6 +514,18 @@ If the email is associated with a specific user, e.g. a notification email, the
 well, otherwise it will be ``None``.
 """
 
+global_email_filter = django.dispatch.Signal(
+    providing_args=['message', 'user']
+)
+"""
+This signal allows you to implement a middleware-style filter on all outgoing emails. You are expected to
+return a (possibly modified) copy of the message object passed to you.
+
+This signal is called on all events and even if there is no known event. ``sender`` is None.
+The ``message`` argument will contain an ``EmailMultiAlternatives`` object.
+If the email is associated with a specific user, e.g. a notification email, the ``user`` argument will be passed as
+well, otherwise it will be ``None``.
+"""
 
 layout_text_variables = EventPluginSignal()
 """