We found this issue only at one account.

The account has 7 months. This is not too big.

He can send emails only to 1 address. The email is sent, and then is showed at sent folder.
But, if he send emails to several address (3 or 4) on one email, the emails are showed as sent, but is not delivered.

We made tests on webmail, by EWS (em Client) on the same PC, on my PC by webmail. 
I can send normally.
I've been digging on logs, but nothing is on them. I found my mails ad delivery logs. But there is nothing related to the emails sent from this crazy account (sales manager account)
This account made me mad :(

Bellow is the log for emails sent by my acoount
Crazy Acoount.txt