In a hybrid mode with Exchange 2010 and when trying to send emails to on premises users the email was not getting delivered. No bouncebacks or errors, just no delivery.
I checked out the connector from 365 to on premises and when validating the connector by setting an email address of an on premises user, the validation failed with a STARTTLS error.
The problem was I had not assigned my webmail.domain.com certificate to the SMTP service in Exchange.
After assigning the certificate – Server Configuration – Right click the webmail.domain.com certificate – Assign to services – SMTP; email started being delivered to my on premises users.