I'm developing a separate service, one of its many modules will process incoming messages, something like a ticket system. We need a module that will be a mail service (linux + iRedMail) and it must send all incoming letters to another complex module for processing and structuring data, taking into account many user settings, subsequent notification, preparation of the data necessary for it, and the like. It turns out: a letter arrived, iRedMail sent it with a POST request to the specified URL, my server saved it in the required format and launched a chain of other necessary services and algorithms for further processing.

The task is as follows. There are users, who have their mail, which receives letters from their users (there may be more than one), for example, user1@company1.com and user2@company1.com. This user registers in our service, and we create a new mailbox for him (for him it is virtual since he will not have access to it as such), for example, guid1@test.com. In their mailboxes, user1@company1.com and user2@company1.com configure mail forwarding to my guid1@test.com. Then, when letters arrive at the guid1@test.com mailbox (which is deployed on iRedMail), they are sent to my service, where I work with them and use them in a form convenient for it. In this case, I need all the capabilities of the mail service to check spam, etc.

If mail is stored not in a database, but in the form of files, this is even better. Then the following scenario is possible: a new letter has arrived, I give access to file storage (for example, Blob Files Azure), this letter is saved in the InboxTemp folder with a unique name (guid), and if there are attached files, they are also assigned a unique name (guid). A POST request is then sent to my site that simply says to create a new email named GUID1 with GUID2/GUID3 attached.

As a result, it should be a full-fledged mail service on which any other mail services can send letters.
It should score 10 out of 10 on all tests.