$ cat /etc/exim4/conf.d/router/100_test-smart
smarthost_for_example1_com:
driver = manualroute
domains = ! +local_domains
senders = *@example1.com
transport = remote_smtp_smarthost
route_list = * smtp.gmail.com

$ sudo exim -f user@example2.com -bt user@ya.ru
R: dnslookup for user@ya.ru
user@ya.ru
  router = dnslookup, transport = remote_smtp
  host mx.yandex.ru [2a02:6b8::89]   MX=10
  host mx.yandex.ru [77.88.21.89]    MX=10
  host mx.yandex.ru [93.158.134.89]  MX=10
  host mx.yandex.ru [87.250.250.89]  MX=10
  host mx.yandex.ru [213.180.193.89] MX=10
  host mx.yandex.ru [213.180.204.89] MX=10

$ sudo exim -f user@example1.com -bt user@ya.ru
user@ya.ru
  router = smarthost_for_example1_com, transport = remote_smtp_smarthost
  host gmail-smtp-msa.l.google.com [2a00:1450:4010:c05::6d]
  host gmail-smtp-msa.l.google.com [173.194.220.109]
  host gmail-smtp-msa.l.google.com [173.194.220.108]