Send E-mail to non-local addresses fails, except for superuser

So here's a weird one. Everything works fine when I'm logged in as the Drupal superuser. The following error applies to all other users with identical permissions. Applies to CiviCRM 1.3 4339 and 4412 on PHP 4x and MySQL 4.1x.

Sending e-mail to contacts, using the "Send E-mail to Contacts" task, fails when non-local e-mail addresses are included in the list of recipients. By non-local, I mean addresses that are not serviced by the server in the group of servers hosting the CiviCRM installation.

Upon attempting to send e-mail to non-local addresses (clicking Send on the compose screen), the user sees the following non-recoverable error:

error

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] =>
[message] => Invalid response code received from server
[mode] => 16
[debug_info] =>
[type] => pear_error
[user_info] =>
[to_string] => [pear_error: message="Invalid response code received from server" code=0 mode=callback callback=CRM_Core_Error::handle prefix="" info=""]
)

To my knowledge nothing has changed on the server re: mail, and previous versions (1.2) of CiviCRM can still send e-mail to non-local addresses.