Set MAILNAME env variable to FQDN hostname for matrix-mailer.

roles/matrix-server/templates/env/env-mailer.j2

@@ -1,4 +1,4 @@
-MAILNAME=matrix-mailer
+MAILNAME={{ hostname_matrix }}
 {% if matrix_mailer_relay_use %}
 RELAYHOST={{ matrix_mailer_relay_host_name }}:{{ matrix_mailer_relay_host_port }}
 {% endif %}