Following steps to reproduce the issue:

1. Activate sendinblue modul
2. Create a new mailing
3. Delete the new mailing

There is no error message but the mailing is not deleted.

Problem is (from my point of view without knowing the source code too good) that in the interface script htdocs/custom/sendinblue/core/triggers/interface_99_modsendinblue_sendinbluetrigger.class.php in line 132 the method $sendinblue->fetch_by_mailing($object->id) is called. This method trusts that there is an entry in llx_sendinblue table with fk_mailing is the id of the new mailing. But there is no such entry. Therefor the sql-statement ends with WHERE= and fails. Then the trigger gives back -1 as result and the core code of Dolibarr does a rollback of the DB