Use different SMTP servers for different domains?

Thanks, agent, for pointing me in the right direction.

In case others need this, the solution was to use hook_domainconf() to expand the "Domain site settings" at
admin/build/domain/conf/2

so you get SMTP settings for the additional domains. I put this in a custom module, mymodule.module, copying almost directly the form from smtp.module:

function mymodule_domainconf() {
  $form['smtp'] = array(
    '#type'  => 'fieldset',
    '#title' => t('SMTP Authentication Support'),
    '#collapsible' => true,
  );
  $form['smtp']['server'] = array(
    '#type'  => 'fieldset',
    '#title' => t('SMTP server settings'),
  );
  $form['smtp']['server']['smtp_host'] = array(
    '#type'          => 'textfield',
    '#title'         => t('SMTP server'),
    '#default_value' => variable_get('smtp_host', ''),
    '#description'   => t('The address of your outgoing SMTP server.'),
  );
  $form['smtp']['server']['smtp_hostbackup'] = array(
    '#type'          => 'textfield',
    '#title'         => t('SMTP backup server'),
    '#default_value' => variable_get('smtp_hostbackup', ''),
    '#description'   => t('The address of your outgoing SMTP backup server. If the primary server can\'t be found this one will be tried. This is optional.'),
  );
  $form['smtp']['server']['smtp_port'] = array(
    '#type'          => 'textfield',
    '#title'         => t('SMTP port'),
    '#size'          => 6,
    '#maxlength'     => 6,
    '#default_value' => variable_get('smtp_port', '25'),
    '#description'   => t('The default SMTP port is 25, if that is being blocked try 80. Gmail uses 465. See !url for more information on configuring for use with Gmail.', array('!url' => l(t('this page'), 'http://gmail.google.com/support/bin/answer.py?answer=13287'))),
  );
  // Only display the option if openssl is installed.
  if (function_exists('openssl_open')) {
    $encryption_options = array(
      'standard' => t('No'),
      'ssl'      => t('Use SSL'),
      'tls'      => t('Use TLS'),
    );
    $encryption_description = t('This allows connection to an SMTP server that requires SSL encryption such as Gmail.');
  }
  // If openssl is not installed, use normal protocol.
  else {
    $encryption_options = array('standard' => t('No'));
    $encryption_description = t('Your PHP installation does not have SSL enabled. See the !url page on php.net for more information. Gmail requires SSL.', array('!url' => l(t('OpenSSL Functions'), 'http://php.net/openssl')));
  }
  $form['smtp']['server']['smtp_protocol'] = array(
    '#type'          => 'select',
    '#title'         => t('Use encrypted protocol'),
    '#options'       => $encryption_options,
    '#default_value' => variable_get('smtp_protocol', 'standard'),
    '#description'   => $encryption_description,
  );

  $form['smtp']['auth'] = array(
    '#type'        => 'fieldset',
    '#title'       => t('SMTP Authentication'),
    '#description' => t('Leave blank if your SMTP server does not require authentication.'),
  );
  $form['smtp']['auth']['smtp_username'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Username'),
    '#default_value' => variable_get('smtp_username', ''),
    '#description'   => t('SMTP Username.'),
  );
  $form['smtp']['auth']['smtp_password'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Password'),
    '#default_value' => variable_get('smtp_password', ''),
    '#description'   => t('SMTP password. Leave blank if you don\'t wish to change it.'),
  );

  $form['smtp']['email_options'] = array(
    '#type'  => 'fieldset',
    '#title' => t('E-mail options'),
  );
  $form['smtp']['email_options']['smtp_from'] = array(
    '#type'          => 'textfield',
    '#title'         => t('E-mail from address'),
    '#default_value' => variable_get('smtp_from', ''),
    '#description'   => t('The e-mail address that all e-mails will be from.'),
  );
  $form['smtp']['email_options']['smtp_fromname'] = array(
    '#type'          => 'textfield',
    '#title'         => t('E-mail from name'),
    '#default_value' => variable_get('smtp_fromname', ''),
    '#description'   => t('The name that all e-mails will be from. If left blank will use the site name of: ') . variable_get('site_name', 'Drupal powered site'),
  );

  return $form;
}

Alternatively, I guess you could also hack the smtp module and put the above function at the end of the file sites/all/modules/smtp.module as function smtp_domainconf()

On a side note I noticed that one has to visit admin/build/domain/conf/2 from the domain one is about to configure. Otherwise it will load the wrong values into the form. For example, if you are configuring SMTP settings for domain-two.com, you should visit:
http://domain-two.com/admin/build/domain/conf/2

and not
http://domain-one.com/admin/build/domain/conf/2

Environment:

• Domain Access 6.x-2.5
• SMTP Authentication Support 6.x-1.x-dev