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.

mox-/lookup.go

@@ -2,7 +2,6 @@ package mox
 
 import (
 	"errors"
-	"fmt"
 	"strings"
 
 	"github.com/mjl-/mox/config"
@@ -12,13 +11,13 @@ import (
 
 var (
 	ErrDomainNotFound  = errors.New("domain not found")
-	ErrAccountNotFound = errors.New("account not found")
+	ErrAddressNotFound = errors.New("address not found")
 )
 
 // FindAccount looks up the account for localpart and domain.
 //
-// Can return ErrDomainNotFound and ErrAccountNotFound.
-func FindAccount(localpart smtp.Localpart, domain dns.Domain, allowPostmaster bool) (accountName string, canonicalAddress string, dest config.Destination, rerr error) {
+// Can return ErrDomainNotFound and ErrAddressNotFound.
+func LookupAddress(localpart smtp.Localpart, domain dns.Domain, allowPostmaster, allowAlias bool) (accountName string, alias *config.Alias, canonicalAddress string, dest config.Destination, rerr error) {
 	if strings.EqualFold(string(localpart), "postmaster") {
 		localpart = "postmaster"
 	}
@@ -39,49 +38,48 @@ func FindAccount(localpart smtp.Localpart, domain dns.Domain, allowPostmaster bo
 	// Check for special mail host addresses.
 	if localpart == "postmaster" && postmasterDomain() {
 		if !allowPostmaster {
-			return "", "", config.Destination{}, ErrAccountNotFound
+			return "", nil, "", config.Destination{}, ErrAddressNotFound
 		}
-		return Conf.Static.Postmaster.Account, "postmaster", config.Destination{Mailbox: Conf.Static.Postmaster.Mailbox}, nil
+		return Conf.Static.Postmaster.Account, nil, "postmaster", config.Destination{Mailbox: Conf.Static.Postmaster.Mailbox}, nil
 	}
 	if localpart == Conf.Static.HostTLSRPT.ParsedLocalpart && domain == Conf.Static.HostnameDomain {
 		// Get destination, should always be present.
 		canonical := smtp.NewAddress(localpart, domain).String()
-		accAddr, ok := Conf.AccountDestination(canonical)
-		if !ok {
-			return "", "", config.Destination{}, ErrAccountNotFound
+		accAddr, a, ok := Conf.AccountDestination(canonical)
+		if !ok || a != nil {
+			return "", nil, "", config.Destination{}, ErrAddressNotFound
 		}
-		return accAddr.Account, canonical, accAddr.Destination, nil
+		return accAddr.Account, nil, canonical, accAddr.Destination, nil
 	}
 
 	d, ok := Conf.Domain(domain)
 	if !ok || d.ReportsOnly {
 		// For ReportsOnly, we also return ErrDomainNotFound, so this domain isn't
 		// considered local/authoritative during delivery.
-		return "", "", config.Destination{}, ErrDomainNotFound
+		return "", nil, "", config.Destination{}, ErrDomainNotFound
 	}
 
-	localpart, err := CanonicalLocalpart(localpart, d)
-	if err != nil {
-		return "", "", config.Destination{}, fmt.Errorf("%w: %s", ErrAccountNotFound, err)
-	}
+	localpart = CanonicalLocalpart(localpart, d)
 	canonical := smtp.NewAddress(localpart, domain).String()
 
-	accAddr, ok := Conf.AccountDestination(canonical)
-	if !ok {
-		if accAddr, ok = Conf.AccountDestination("@" + domain.Name()); !ok {
+	accAddr, alias, ok := Conf.AccountDestination(canonical)
+	if ok && alias != nil && allowAlias {
+		return "", alias, canonical, config.Destination{}, nil
+	} else if !ok {
+		if accAddr, alias, ok = Conf.AccountDestination("@" + domain.Name()); !ok || alias != nil {
 			if localpart == "postmaster" && allowPostmaster {
-				return Conf.Static.Postmaster.Account, "postmaster", config.Destination{Mailbox: Conf.Static.Postmaster.Mailbox}, nil
+				return Conf.Static.Postmaster.Account, nil, "postmaster", config.Destination{Mailbox: Conf.Static.Postmaster.Mailbox}, nil
 			}
-			return "", "", config.Destination{}, ErrAccountNotFound
+			return "", nil, "", config.Destination{}, ErrAddressNotFound
 		}
 		canonical = "@" + domain.Name()
 	}
-	return accAddr.Account, canonical, accAddr.Destination, nil
+	return accAddr.Account, nil, canonical, accAddr.Destination, nil
 }
 
 // CanonicalLocalpart returns the canonical localpart, removing optional catchall
 // separator, and optionally lower-casing the string.
-func CanonicalLocalpart(localpart smtp.Localpart, d config.Domain) (smtp.Localpart, error) {
+func CanonicalLocalpart(localpart smtp.Localpart, d config.Domain) smtp.Localpart {
 	if d.LocalpartCatchallSeparator != "" {
 		t := strings.SplitN(string(localpart), d.LocalpartCatchallSeparator, 2)
 		localpart = smtp.Localpart(t[0])
@@ -90,5 +88,24 @@ func CanonicalLocalpart(localpart smtp.Localpart, d config.Domain) (smtp.Localpa
 	if !d.LocalpartCaseSensitive {
 		localpart = smtp.Localpart(strings.ToLower(string(localpart)))
 	}
-	return localpart, nil
+	return localpart
+}
+
+// AllowMsgFrom returns whether account is allowed to submit messages with address
+// as message From header, based on configured addresses and membership of aliases
+// that allow using its address.
+func AllowMsgFrom(accountName string, msgFrom smtp.Address) bool {
+	accName, alias, _, _, err := LookupAddress(msgFrom.Localpart, msgFrom.Domain, false, true)
+	if err != nil {
+		return false
+	}
+	if alias != nil && alias.AllowMsgFrom {
+		for _, aa := range alias.ParsedAddresses {
+			if aa.AccountName == accountName {
+				return true
+			}
+		}
+		return false
+	}
+	return accName == accountName
 }