Add sender/receiver model

docs/first_steps.md

@@ -133,3 +133,38 @@ server {
 }
 ```
 
+# Sender and receiver model
+
+When a mailbox is created, a user is allowed to send mail from and receive mail for his own mailbox address.
+
+    Mailbox me@example.org is created. example.org is a primary domain. 
+    Note: a mailbox cannot be created in an alias domain.
+
+    me@example.org is only known as me@example.org.
+    me@example.org is allowed to send as me@example.org.
+
+We can add an alias domain for example.org:
+
+    Alias domain alias.com is added and assigned to primary domain example.org.
+    me@example.org is now known as me@example.org and me@alias.com.
+    me@example.org is now allowed to send as me@example.org and me@alias.com.
+
+We can add aliases for a mailbox to receive mail for and to send from this new address.
+
+It is important to know, that you are not able to receive mail for `my-alias@my-alias-domain.tld`. You would need to create this particular alias.
+
+    me@example.org is assigned the alias alias@example.org
+    me@example.org is now known as alias@example.org, me@alias.com, alias@example.org
+
+    me@example.org is NOT known as alias@alias.com.
+
+Administrators and domain administrators can edit mailboxes to allow specific users to send as other mailbox users ("delegate" them).
+
+You can choose between mailbox users or completely disable the sender check for domains.
+
+**SOGo "mail from" addresses**
+
+Mailbox users can, obviously, select their own mailbox address, as well as all alias addresses and aliases that exist through alias domains.
+
+If you want to select another _existing_ mailbox user as your "mail from" address, this user has to delegate you access through SOGo (see SOGo documentation). Moreover a mailcow (domain) administrator
+needs to grant you access as described above.