Exim4 Smart Host Relay

I am running Exim 4.71. I want to:

1. Route all email from A.com through mail.A.com
2. Route all email from [B-E].com through mail.B.com
3. Send all other email directly.

Here is the configuration I have that doesn't work like I hoped:

domainlist a_domains = a.com
domainlist b_domains = b.com : c.com : d.com : e.com

begin routers

smart_route_a:
  driver = manualroute
  domains = +a_domains
  transport = remote_smtp
  route_list = +a_domains mail.a.com
  no_more

smart_route_b:
  driver = manualroute
  domains = +b_domains
  transport = remote_smtp
  route_list = +b_domains mail.mollenhour.com
  no_more

dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
  no_more

When I send an email e.g. with PHP's mail() or Zend_Mail_Transport_Smtp setting both From: and Return-Path: as user@a.com, the smart_route_a router is not used, the dnslookup is used instead. Disabling dnslookup results in no mail being sent.

From the logs it appears that email sent to someone@a.com uses smart_route_a, but the same email sent from user@a.com to user@gmail.com is sent using dnslookup.

How do I make email from user@a.com be relayed via mail.a.com?