Incoming mail gateway

I have a requirement for an inbound mail gateway. The gateway ideally runs as a very lightweight service that receives standard SMTP queries from external mail providers. It ideally has the following features; - the facility to execute a logic app (such as Lamda or Azure Functions) to determine if to catch and hold the e-mail for later release - the facility to store held e-mails in an enterprise database system, like Microsoft SQL server with a unique identified - the facility to reply as the intended recipient in HTML format - the facility to release held e-mails via API or function call using the unique ID - the facility to hold verbose logging of any and all activity on the service - the facility to scale and accommodate 1000's of e-mails at any given time - the facility to be architected in such a way that it would act as the primary MX host, but relay to an underlying e-mail provider, like 365 or gMail - tested and demonstrated working end to end - considerations around SPF configuration on domain Open to recommendations on technology stack and model, however preference for serverless architecture and Azure/AWS/GCP self sustaining. Thank you