Rebalance Mailman's and Exim's outgoing batch size

In order to avoid tripping Exim's threshold for punting deliveries
to periodic queue processing, make sure to set Mailman's
mta.max_recipients value less than Exim's smtp_accept_max_per_host.
This should eliminate the "no immediate delivery: more than X
messages received in one connection" errors in Exim's mainlog.

While we're at it, increase both for greater throughput.

Change-Id: I9e21eb19b85c9a4b19f5e6c5d03a89c89028bbc4

inventory/service/group_vars/mailman3.yaml

@@ -25,7 +25,8 @@ borg_backup_excludes_extra:
 exim_queue_interval: '1m'
 exim_queue_run_max: '50'
 exim_smtp_accept_max: '100'
-exim_smtp_accept_max_per_host: '10'
+# This should be set to more than mailman's mta.max_recipients value.
+exim_smtp_accept_max_per_host: '50'
 exim_routers:
   - mailman_verp_router: |
       {% raw -%}

playbooks/roles/mailman3/templates/mailman-extra.cfg.j2

@@ -5,6 +5,10 @@
 # a human.
 site_owner: "root@{{ ansible_fqdn }}"
 
+# We set this to less than the Exim's smtp_accept_max_per_host value.
+[mta]
+max_recipients: 45
+
 # The outgoing runner is what is responsible for sending email. By default
 # there is only one instance of this runner. We have seen some emails get
 # queued up for 5-10 minutes in lists with larger membership. Increase this