Enable sender address verification in Exim

Now that we don't use satellite config anymore it can make a
difference.

ansible/roles/mailman/templates/exim4.conf.template

@@ -15,6 +15,7 @@ domainlist mm_domains = {{ list_domain }}
 MM_WRAP=MM_HOME/mail/mailman
 MM_LISTCHK=MM_HOME/lists/${lc::$local_part}/config.pck
 
+CHECK_RCPT_VERIFY_SENDER = 1
 spamd_address = rspamd.mail.einfra.hu 11333 variant=rspamd
 
 {% raw -%}