Understanding SEA logs : message sent or not ?

Hello,

I have an Odoo (Python code) mailing list server sending emails through SEA. Most of them are delivered without problem, but some emails seems not delivered to their recipient for an unknow reason (unknow to me at this stage). I'm trying to understand the SEA logs.

- On the odoo server log I have an indication that the mail is successfuly sent to the SEA

May 30 12:40:32 odoo Odoo Server 8.0:REDACTED-DBNAME:INFO:openerp.addons.mail.mail_mail:Mail with ID 8451890 and Message-Id u'<1590833855.093642950057983.346811027420296-REDACTED-reply_to@odoo>' successfully sent

- On the SEA Ihave this for the mail with the same MESSAGE-ID:

and if I click on "view log details" I get this :

2020-05-30 12:40:32 castor postfix/smtpd[31179]: 301461EF775A_ED23820F: client=douve.ecolo.be[79.99.203.253]
2020-05-30 12:40:32 castor postfix/cleanup[31186]: 301461EF775A_ED23820F: message-id=<1590833855.093642950057983.346811027420296-openerp-reply_to@odoo>
2020-05-30 12:40:32 castor postfix/qmgr[28504]: 301461EF775A_ED23820F: from=<diffusion-bounce+8451890-email.coordinate-39566@ecolo.be>, size=101992, nrcpt=1 (queue active)


What does it mean ? That the incoming mail was not delivered ? (to a gmail address)
And how can I know why ? 
And what could explain that the oddo server think the mail was successfully relayed to the SEA ?

The communication between the oddoo server and the SEA is with the SMTP port 25 without authentification
The odoo server IP is in the "Allowed hosts list"
Most emails sent from the odoo server are sent exactly the same way without problems but it seems not all of them.
An indication could be that there is a high volume

Has someone an explanation for the log, and for the possible causes ?

Thanks in advance,

Vincent