Well, unfortunately how else could we troubleshoot and provide some more feedback information about the issue?
May I ask if you are using WordPress in such case?
Is the SMTP plugin configured as a “general sent out” emails, meaning it would sent out emails via origin host/server per configured hostname like mail.domain.com?
Are you also using a separate plugin like Mailchimp for the “Brevo” service?
Therefore, meaning any email sent out from WordPress would ignore the “brevo” SMTP hostname, also the DKIM, and just use the mail.domain.com as SMTP handler.
In other words, if you’re using Google Workspace it’ll be you’d be using Google SMTP (wrong one?) instead of the correct “brevo” one for sending emails?
For sure you can use both, it’s just a bit tricky as it seems to configure the settings and you’d have to have all the DNS records in place, if so.