Using SES as an SMTP relay with Office365

Working on a DC migration. As part of this there are applications that need to send emails and currently do via onprem SMTP relays (Exchange) How can this be achieved with SES + O365?

SES provides the option of dedicated IP address which can be allowed but how does this work with the domain namespace where the sending address will have the same domain as the O365 tenancy. E.g. SES would be configured with the namespace of "customer.com" and O365 is also configured with the namespace of "customer.com" as they want the applications emails to come from one of their email addresses. The above config will result in a phishing attack being detected. Also there is a requirement that these emails are treated as internal emails which do not get scanned and vetted as do external emails. E.g. internal emails are allowed a larger attachment size than external emails from printer scans, document applications etc. There are connectors that can be configured but I was wondering if anyone has done this and understands the config required on the O365 side. Thanks for your help.