Hi Curtis, thank you for your answer !
But I'm confused about where and why use certificates in this section.
I have nothing in Settings/SSL Certificates.
But I do have a wildcard certificate for my main mailserver domain, and it is used in the Settings/Bindings/Port section :
(This certificate is also installed in IIS for https connections to the webmail, all users use the same URL)
I never paid attention until yesterday to this settings in Domain options :
I enabled TLS for the domain with problem, I saw in the Delivery log there seems to be a new dialog now when connecting to Gmail :
[2024.09.30] 15:13:08.715 [62123979] CMD: STARTTLS
[2024.09.30] 15:13:08.746 [62123979] RSP: 220 2.0.0 Ready to start TLS
But I still don't get if this is what was missing, and which certificate is used (if any) in this case...
And again, for me it is mymailserver.com that is talking to Gmail (so certificate should be on this) and not mycustomer.com, so does he need to have a certificate for his domain ?