error trying to sent a delivery from a sandbox AC

Hi all,

When I try to sent a delivery, from my sandbox, I get this error in the "mtachild.log" :

2019-12-16 20:43:48.187+01 00002728 00002804 1 info log Starting an MTA child (pid=10024, tid=10244)...
2019-12-16 20:45:44.108+01 00002728 00002804 1 info log ======== Deferred Queue (1) ========
2019-12-16 20:45:44.109+01 00002728 00002804 1 info log 1 localhost
2019-12-16 20:45:44.109+01 00002728 00002804 1 info log =====================================

am I missing something in the configuration ?

Thanks.