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+# design #
+
+## overview #
+----------------------
+This page pertains to the incoming mail exchange servers of the provider.
+
+General overview of how incoming email will work:
+
+ 1. Incoming message is received by provider's MX servers.
+ 2. The MTA (postfix in our case) does a ton of checks on the message before we
+ even check to see if the recipient is valid (this comes from experience
+ running the riseup mail infrastructure, where the vast majority of messages
+ can be rejected early in the SMTP reception and thus save a ton of processing
+ time on the server).
+ 3. Postfix then queries the database to check if the recipient is valid, if
+ they are over quota, if their account is enabled, and to resolve any aliases
+ for the account.
+ 4. The message is then delivered to an on-disk message spool.
+ 5. A daemon watches for new files in this spool. Each message is encrypted to
+ the user's public key, and stored in the user's incoming message queue (stored
+ in couchdb), and removed from disk.
+ 6. When the user next logs in with their client, the user's message queue is
+ emptied by the client.
+ 7. Each message is decrypted by the client, and then stored in the user's
+ "inbox" as an unread message.
+ 8. This local inbox uses soledad for storage
+ 9. Soledad, in the background, will then re-encrypt this email (now a soledad
+ document), and sync to the cloud.
+
+## postfix pipeline ##
+---------------------------
+incoming mx servers will run postfix, configured in a particular way:
+
+ 1. postscreen: before accepting an incoming message, checks RBLs, checks RFC
+ validity, checks for spam pipelining.
+    (pass) proceed to next step.
+    (fail) return SMTP error, which bounces email.
+ 2. more SMTP checks: valid hostnames, etc.
+    (pass) accepted, proceed to next step.
+    (fail) return SMTP error, which bounces email.
+
+ 3. check_recipient_access -- look up each recipient and ensure they are
+ allowed to receive messages.
+    (pass) empty result, proceed to next step.
+    (fail) return SMTP error code and error comment, bounce message.
+ 4. milter processessing (spamassassin & clamav)
+    (pass) continue
+    (fail) bounce message, flag as spam, or silently kill.
+ 5. virtual_alias_maps -- map user defined aliases and forwards
+    (local address) continue if new address is for this mx
+    (remote address) continue. normally, postfix would relay to the remote domain, but we don't want that.
+ 6. deliver message to spool
+    (write) save the message to disk on the mx.
+ 7. postfix's job is done, mail_receiver picks up email from spool directory
+
+Questions:
+
+ * what is the best way to have postfix write a message to a spool directory?
+   There is a built-in facility for saving to a maildir, so we could just
+   specify a common maildir for everyone. alternately, we could pipe to a
+   simple command that was responsible for safely saving the file to disk. a
+   third possibility would be to have a local long running daemon that spoke
+   lmtp that postfix forward the message on to for delivery.
+
+ * if virtual_alias_maps comes after check_recipient_access, then a user with
+   aliases set but who is over quota will not be able to forward email. i think
+   this is fine.
+
+ * if we are going to support forwarding, we should ensure that the message
+   gets encrypted before getting forwarded. so, postfix should not do any
+   forwarding. instead, this should be the job of mail_receiver.
+
+Considerations:
+
+ 1. high load should fill queue, not crash pipeline: It is important that the
+ pipeline be able to handle massive bursts of email, as often happens with
+ email. This means map lookups need to be very fast, and when there is a high
+ load of email postfix should not be waiting on the mail receiver but must be
+ able to pass the message off quickly and have the slower mail receiver churn
+ through the backlog as best it can.
+
+ 2. don't lose messages: It is important to not lose any messages when there is
+ a problem. So, generally, a copy of an email should always exist in some spool
+ somewhere, and that copy should not be deleted until there is confirmation
+ that the next stage has succeeded.
+
+## alias_resolver ##
+------------------------------
+The alias_resolver will be a daemon running on MX servers that handles lookups
+in the user database of email aliases, forwards, quota, and account status.
+
+Communication with:
+
+ 1. postfix:: alias_resolver will be bound to localhost and speak postfix's
+ very simple [tcp map protocol -> http://www.postfix.org/tcp_table.5.html].
+
+ 2. couchdb:: alias_resolver will make couchdb queries to a local http load
+ balancer that connects to a couchdb/bigcouch
+ cluster. [directly accessing the couch->https://we.riseup.net/leap+platform/querying-the-couchdb]
+ might help getting started.
+
+Discussion:
+
+ * we want the lookups to be fast. using views in couchdb, these should be very
+   fast. when using bigcouch, we can make it faster by specifying a read quorum
+   of 1 (instead of the default 2). this will make it so that only a single
+   couchdb needs to be queried to find the result. i don't know if this would
+   cause problems, but aliases don't change very often.
+
+alias_resolver will be responsible for two map lookups in postfix:
+
+### check_recipient ###
+
+postfix config:
+
+@check_recipient_access tcp:localhost:1000@
+
+postfix will send "get username@domain.org" and alias_resolver should return an
+empty result ("200 \n", i think) if postfix should deliver email to the
+user. otherwise, it should return an error. here is example response, verbatim,
+that can be used to bounce over quota users:
+
+```
+200 DEFER_IF_PERMIT Sorry, your message cannot be delivered because the
+recipient's mailbox is full. If you can contact them another way, you may wish
+to tell them of this problem.  
+```
+
+"DEFER_IF_PERMIT" will let the other MX known that this error is temporary and
+that they should try again soon. Typically, an MX will try repeatedly, at
+longer and longer intervals, for four days before giving up.
+
+### virtual alias map ###
+
+postfix config:
+
+@virtual_alias_map tcp:localhost:1001@
+
+postfix will send "get alias-address@domain.org" and alias_resolver should
+return "200 id_123456\n", where 123456 is the unique id of the user that has
+alias-address@domain.org.
+
+couchdb should have a view that will let us query on an (alias) address and
+return the user id.
+
+note: if the result of the alias map (e.g. id_123456) does not have a domain
+suffix, i think postfix will use the 'default transport'. if we want it to use
+the virtual transport instead, we should append the domain (eg
+id_123456@example.org). see
+http://www.postfix.org/ADDRESS_REWRITING_README.html#resolve
+
+## mail_receiver ##
+
+the mail_receiver is a daemon that runs on incoming MX servers and is
+responsible for encrypting incoming email to the user's public key and saving
+the email to an incoming queue database for that user.
+
+communicates with:
+
+ * message spool directory:: mail_reciever sits and waits for new email to be
+   written to the spool directory (maybe using this
+   https://github.com/seb-m/pyinotify, i think it is better than FAM). when a
+   new file is dumped into the spool, mail_receiver reads the file, encrypts
+   the entire thing using the public key of the recipient, and saves to
+   couchdb.
+ * couchdb get:: mail_receiver does a query on user id to get back user's
+   public openpgp key. read quorum of 1 is probably ok.
+ * couchdb put:: mail_receiver communicates with couchdb for storing encrypted
+   email for each user (eventually, mail_receiver will communicate with a local
+   http proxy, that communicates with a bigcouch cluster, but the api is
+   identical)
+
+discussion:
+ * i am not sure if postfix adds a header to indicate to whom a message was
+   actually delivered. if not, this is a problem, because then how do we know
+   what db to put it in or what public key to use? this is perhaps a good
+   reason to not let postfix handle writing the message to disk, but instead
+   pipe it to another command (because postfix sets env variables for stuff
+   like recipient).
+
+ * should the incoming message queue be a separate database or should it be
+   just documents in the user's main database with special flags?
+
+ * whenever possible, we should refer to the user by a fixed id, not their
+   username, because we want to support the ability to change usernames. so,
+   for example, database names should not be based on usernames.