drupal forgets how to use amazon ses to send mail

Twice now I've noticed that Drupal has stopped using Amazon SES to send mail even though the module is enabled and "Turn this module on or off" is "On".

When I use the settings page to send a test email, it tells me it was successful but does not use Amazon SES to send.

When I write a script to send a test message using amazon_ses_send(), it goes through fine - so it is not a problem with the account.

When I disable the module and re-enable it and set the "turn this module on or off" setting back to "On", it uses SES to send again.

I can't find any reason for this happening. Is anyone else having this kind of trouble?

Is there anything I can do to keep Drupal from reverting to its old self? Would a module weight help, perhaps?