mail: implement aliases across domains

part of #577

migrations/schema.sql

@@ -177,13 +177,15 @@ CREATE TABLE IF NOT EXISTS mailboxes(
     name VARCHAR(128) NOT NULL,
     type VARCHAR(16) NOT NULL, /* 'mailbox', 'alias', 'list' */
     ownerId VARCHAR(128) NOT NULL, /* user id */
-    aliasTarget VARCHAR(128), /* the target name type is an alias */
+    aliasName VARCHAR(128), /* the target name type is an alias */
+    aliasDomain VARCHAR(128), /* the target domain */
     membersJson TEXT, /* members of a group. fully qualified */
     membersOnly BOOLEAN DEFAULT false,
     creationTime TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     domain VARCHAR(128),
 
     FOREIGN KEY(domain) REFERENCES mail(domain),
+    FOREIGN KEY(aliasDomain) REFERENCES mail(domain),
     UNIQUE (name, domain));
 
 CREATE TABLE IF NOT EXISTS subdomains(