Error message on sending a mailing, CREATE TEMPORARY TABLE
The last time I saw something like this was when custom fields on a table had been added, a group had been created using these custom fields and subsequently the custom fields had physically been deleted. This meant the custom group cache table failed to create - the group used in the mailing was the culprit. I'd try using a different group as a test. If it works then look to rebuild your smart group - the full error in the SQL should point you in the right direction.