mailing tests fail with error 10005 but outgoing SMTP tests succeed

if I use the outgoing SMTP email test the email is delivered properly to the right recipient.

If I try to setup a mailing under civimail, when I get to the 'test' step, it fails with the following message:

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

[code] => 10005
[message] => Failed to add recipient: johnedstrom@firebare.org [SMTP: Invalid response code received from SMTP server while sending email. This is often caused by a misconfiguration in Outbound Email settings. Please verify the settings at Administer CiviCRM >> Global Settings >> Outbound Email (SMTP). (code: 550, response: Verification failed for <b.54.59.9247bdda21c81e60@firebare.org>
No Such User Here
Sender verify failed)]
[mode] => 16
[debug_info] =>
[type] => PEAR_Error
[user_info] =>
[to_string] => [pear_error: message="Failed to add recipient: johnedstrom@firebare.org [SMTP: Invalid response code received from SMTP server while sending email. This is often caused by a misconfiguration in Outbound Email settings. Please verify the settings at Administer CiviCRM >> Global Settings >> Outbound Email (SMTP). (code: 550, response: Verification failed for <b.54.59.9247bdda21c81e60@firebare.org>
No Such User Here
Sender verify failed)]" code=10005 mode=callback callback=CRM_Core_Error::handle prefix="" info=""]
)

FWIW, we're using drupal 6 & civicrm 4.1.1. I saw the same behavior with 3.4.*. We haven't needed the mailing feature, until now, and so didn't pursue the issue.

The site is hosted on hostmonster, along with our battery of email accounts. We see no problems with our ancillary email accounts when we use other IMAP, POP & SMTP tools to interact with them. I've checked, reset, and double checked the veracity of the account passwords and other details.