add aliases/lists: when sending to an alias, the message gets delivered to all members

the members must currently all be addresses of local accounts.

a message sent to an alias is accepted if at least one of the members accepts
it. if no members accepts it (e.g. due to bad reputation of sender), the
message is rejected.

if a message is submitted to both an alias addresses and to recipients that are
members of the alias in an smtp transaction, the message will be delivered to
such members only once.  the same applies if the address in the message
from-header is the address of a member: that member won't receive the message
(they sent it). this prevents duplicate messages.

aliases have three configuration options:
- PostPublic: whether anyone can send through the alias, or only members.
  members-only lists can be useful inside organizations for internal
  communication. public lists can be useful for support addresses.
- ListMembers: whether members can see the addresses of other members. this can
  be seen in the account web interface. in the future, we could export this in
  other ways, so clients can expand the list.
- AllowMsgFrom: whether messages can be sent through the alias with the alias
  address used in the message from-header. the webmail knows it can use that
  address, and will use it as from-address when replying to a message sent to
  that address.

ideas for the future:
- allow external addresses as members. still with some restrictions, such as
  requiring a valid dkim-signature so delivery has a chance to succeed. will
  also need configuration of an admin that can receive any bounces.
- allow specifying specific members who can sent through the list (instead of
  all members).

for github issue #57 by hmfaysal.
also relevant for #99 by naturalethic.
thanks to damir & marin from sartura for discussing requirements/features.

README.md

@@ -24,10 +24,10 @@ See Quickstart below to get started.
 - Automatic TLS with ACME, for use with Let's Encrypt and other CA's.
 - DANE and MTA-STS for inbound and outbound delivery over SMTP with STARTTLS,
   including REQUIRETLS and with incoming/outgoing TLSRPT reporting.
-- Web admin interface that helps you set up your domains and accounts
-  (instructions to create DNS records, configure
-  SPF/DKIM/DMARC/TLSRPT/MTA-STS), for status information, managing
-  accounts/domains, and modifying the configuration file.
+- Web admin interface that helps you set up your domains, accounts and list
+  aliases (instructions to create DNS records, configure
+  SPF/DKIM/DMARC/TLSRPT/MTA-STS), for status information, and modifying the
+  configuration file.
 - Account autodiscovery (with SRV records, Microsoft-style, Thunderbird-style,
   and Apple device management profiles) for easy account setup (though client
   support is limited).
@@ -135,7 +135,6 @@ https://nlnet.nl/project/Mox/.
 
 ## Roadmap
 
-- Aliases, for delivering to multiple local accounts.
 - Calendaring with CalDAV/iCal
 - More IMAP extensions (PREVIEW, WITHIN, IMPORTANT, COMPRESS=DEFLATE,
   CREATE-SPECIAL-USE, SAVEDATE, UNAUTHENTICATE, REPLACE, QUOTA, NOTIFY,
@@ -145,6 +144,7 @@ https://nlnet.nl/project/Mox/.
 - Forwarding (to an external address)
 - Add special IMAP mailbox ("Queue?") that contains queued but
   undelivered messages, updated with IMAP flags/keywords/tags and message headers.
+- External addresses in aliases/lists.
 - Sieve for filtering (for now see Rulesets in the account config)
 - Autoresponder (out of office/vacation)
 - OAUTH2 support, for single sign on