I was looking for any other log in my server, but all I can see is what I said in the last email. My OTRS system says the message was sent. The account used by OTRS (a google email) also says the email was sent. Here I think there is nothing more to see. I dont know if I can see any MTA log with this scenario.
I did a test with a customer using a google group as email. I did not receive the email. I also checked the spam box. I'm not totally sure if it is an OTRS problem or not, since the customer received the email when he asked for his password.
Thanks for all your help,