📝(dimai) update docs on dimail

update docs on dimail to describe interoperability between la régie
and dimail

docs/interoperability/dimail.md

@@ -0,0 +1,28 @@
+# dimail 
+
+## What is dimail ? 
+
+The mailing solution provided in La Suite is [Open-XChange](https://www.open-xchange.com/) (OX) server. 
+OX not having a provisioning API, 'dimail' or 'dimail-api' was created to allow mail-provisioning through La Régie.
+
+API and its documentation can be found [here](https://api.dev.ox.numerique.gouv.fr/docs#/).
+
+## Architectural links of dimail
+
+As dimail's primary goal is to act as an interface between La Régie and OX, its architecture is similar to th
+
+### Domains
+
+Upon creating a domain on domain manager tab of La Régie, the same domain is created on dimail and will undergo a serie of checks. When a domain passes all the checks, it's considered valid and mailboxes can be created on it. 
+
+### Users
+
+The ones issuing requests. These users will reflect domain owners and administrators on la Régie, for logging purposes and to allow direct use of dimail, if desired.
+
+### Permissions
+
+These are the permissions linking users to domains
+A permission is automatically created when : 
+- upon domain creation : perm for La Régie to manage domain 
+- upon user creation : perm for La Régie to manage user
+- upon access creation, if owner or admin, perm for this user to manage domain