Exim config for working with Google delivery restrictions.

exim4_main_10_local_single_domain

domainlist single_domain_mx = aspmx.l.google.com : gmail-smtp-in.l.google.com

exim4_router_180_local_single_domain

dnslookup_single_domain:
  debug_print = "R: dnslookup_single_domain for $local_part@$domain"
  driver = dnslookup
  domains = ! +local_domains : ! +relay_to_domains
  condition = ${if forany{${lookup dnsdb{>: mxh=$domain}}}{match_domain{$item}{+single_domain_mx}}}
  transport = remote_smtp_single_domain
  same_domain_copy_routing = yes
  no_more

exim4_transport_40_local_single_domain

remote_smtp_single_domain:
  debug_print = "T: remote_smtp_single_domain for $local_part@$domain"
  driver = smtp
  multi_domain = false
  max_rcpt = 8
.ifdef REMOTE_SMTP_HOSTS_AVOID_TLS
  hosts_avoid_tls = REMOTE_SMTP_HOSTS_AVOID_TLS
.endif
.ifdef REMOTE_SMTP_HEADERS_REWRITE
  headers_rewrite = REMOTE_SMTP_HEADERS_REWRITE
.endif
.ifdef REMOTE_SMTP_RETURN_PATH
  return_path = REMOTE_SMTP_RETURN_PATH
.endif
.ifdef REMOTE_SMTP_HELO_DATA
  helo_data=REMOTE_SMTP_HELO_DATA
.endif
.ifdef DKIM_DOMAIN
dkim_domain = DKIM_DOMAIN
.endif
.ifdef DKIM_SELECTOR
dkim_selector = DKIM_SELECTOR
.endif
.ifdef DKIM_PRIVATE_KEY
dkim_private_key = DKIM_PRIVATE_KEY
.endif
.ifdef DKIM_CANON
dkim_canon = DKIM_CANON
.endif
.ifdef DKIM_STRICT
dkim_strict = DKIM_STRICT
.endif
.ifdef DKIM_SIGN_HEADERS
dkim_sign_headers = DKIM_SIGN_HEADERS
.endif
.ifdef TLS_DH_MIN_BITS
tls_dh_min_bits = TLS_DH_MIN_BITS
.endif
.ifdef REMOTE_SMTP_TLS_CERTIFICATE
tls_certificate = REMOTE_SMTP_TLS_CERTIFICATE
.endif
.ifdef REMOTE_SMTP_PRIVATEKEY
tls_privatekey = REMOTE_SMTP_PRIVATEKEY
.endif

This is a minor update to a 2012 exim config for dealing with Google's multi-domain routing restrictions, with the addition of max_rcpt in order to avoid a "domain policy size" restriction which seems to kick-in if you attempt to deliver to more than eight recipients at a time. (Which mailing-list servers will tend to do.)

452-4.5.3 Domain policy size per transaction exceeded, please try this recipient in a separate transaction. unique.string - gsmtp