Changed files:

For authentication from mobile / remote:

/etc/exim4/conf.d/auth/30_exim4-config_examples

This block uncommented (for SASL auth from mobile):

plain_saslauthd_server:
  driver = plaintext
  public_name = PLAIN
  server_condition = ${if saslauthd{{$auth2}{$auth3}}{1}{0}}
  server_set_id = $auth2
  server_prompts = :
  .ifndef AUTH_SERVER_ALLOW_NOTLS_PASSWORDS
  server_advertise_condition = ${if eq{$tls_in_cipher}{}{}{*}}
  .endif

/etc/exim4/conf.d/main/03_exim4-config_tlsoptions

Added at top of file:

MAIN_TLS_ENABLE = yes
tls_on_connect_ports=465

For DKIM

/etc/exim4/conf.d/transport/30_exim4-config_remote_smtp

Add under driver = smtp

  dkim_private_key = /etc/exim4/dkim/disruptive.org.uk-private.pem
  dkim_sign_headers = subject:to:from

/etc/exim4/conf.d/transport/10_exim4-config_transport-macros

Add at top of file:

DKIM_CANON = relaxed
DKIM_SELECTOR = 20151021
DKIM_DOMAIN = disruptive.org.uk
DKIM_FILE = /etc/exim4/dkim/disruptive.org.uk-private.pem

General setup

The file /etc/exim4/update-exim4.conf.conf, created by dpkg-reconfigure exim4-config

# /etc/exim4/update-exim4.conf.conf
#
# Edit this file and /etc/mailname by hand and execute update-exim4.conf
# yourself or use 'dpkg-reconfigure exim4-config'
#
# Please note that this is _not_ a dpkg-conffile and that automatic changes
# to this file might happen. The code handling this will honor your local
# changes, so this is usually fine, but will break local schemes that mess
# around with multiple versions of the file.
#
# update-exim4.conf uses this file to determine variable values to generate
# exim configuration macros for the configuration file.
#
# Most settings found in here do have corresponding questions in the
# Debconf configuration, but not all of them.
#
# This is a Debian specific file

dc_eximconfig_configtype='internet'
dc_other_hostnames='disruptive.org.uk'
dc_local_interfaces='213.138.114.51'
dc_readhost=''
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost=''
CFILEMODE='644'
dc_use_split_config='true'
dc_hide_mailname='false'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'

/etc/mailname

disruptive.org.uk