allow zero configured addresses for an account

preventing writing out a domains.conf that is invalid and can't be parsed
again. this happens when the last address was removed from an account. just a
click in the admin web interface.

accounts without email address cannot log in.

for issue #133 by ally9335

config/config.go

@@ -107,7 +107,7 @@ type SpecialUseMailboxes struct {
 // Dynamic is the parsed form of domains.conf, and is automatically reloaded when changed.
 type Dynamic struct {
 	Domains            map[string]Domain  `sconf-doc:"NOTE: This config file is in 'sconf' format. Indent with tabs. Comments must be on their own line, they don't end a line. Do not escape or quote strings. Details: https://pkg.go.dev/github.com/mjl-/sconf.\n\n\nDomains for which email is accepted. For internationalized domains, use their IDNA names in UTF-8."`
-	Accounts           map[string]Account `sconf-doc:"Account represent mox users, each with a password and one or more email addresses to which email can be delivered (possibly different domains). Each account has its own on-disk directory holding its messages and index database. An account name has is not an email address."`
+	Accounts           map[string]Account `sconf-doc:"Accounts represent mox users, each with a password and email address(es) to which email can be delivered (possibly at different domains). Each account has its own on-disk directory holding its messages and index database. An account name is not an email address."`
 	WebDomainRedirects map[string]string  `sconf:"optional" sconf-doc:"Redirect all requests from domain (key) to domain (value). Always redirects to HTTPS. For plain HTTP redirects, use a WebHandler with a WebRedirect."`
 	WebHandlers        []WebHandler       `sconf:"optional" sconf-doc:"Handle webserver requests by serving static files, redirecting or reverse-proxying HTTP(s). The first matching WebHandler will handle the request. Built-in handlers, e.g. for account, admin, autoconfig and mta-sts always run first. If no handler matches, the response status code is file not found (404). If functionality you need is missng, simply forward the requests to an application that can provide the needed functionality."`
 	Routes             []Route            `sconf:"optional" sconf-doc:"Routes for delivering outgoing messages through the queue. Each delivery attempt evaluates account routes, domain routes and finally these global routes. The transport of the first matching route is used in the delivery attempt. If no routes match, which is the default with no configured routes, messages are delivered directly from the queue."`
@@ -352,7 +352,7 @@ type Account struct {
 	Domain       string                 `sconf-doc:"Default domain for account. Deprecated behaviour: If a destination is not a full address but only a localpart, this domain is added to form a full address."`
 	Description  string                 `sconf:"optional" sconf-doc:"Free form description, e.g. full name or alternative contact info."`
 	FullName     string                 `sconf:"optional" sconf-doc:"Full name, to use in message From header when composing messages in webmail. Can be overridden per destination."`
-	Destinations map[string]Destination `sconf-doc:"Destinations, keys are email addresses (with IDNA domains). If the address is of the form '@domain', i.e. with localpart missing, it serves as a catchall for the domain, matching all messages that are not explicitly configured. Deprecated behaviour: If the address is not a full address but a localpart, it is combined with Domain to form a full address."`
+	Destinations map[string]Destination `sconf:"optional" sconf-doc:"Destinations, keys are email addresses (with IDNA domains). All destinations are allowed for logging in with IMAP/SMTP/webmail. If no destinations are configured, the account can not login. If the address is of the form '@domain', i.e. with localpart missing, it serves as a catchall for the domain, matching all messages that are not explicitly configured. Deprecated behaviour: If the address is not a full address but a localpart, it is combined with Domain to form a full address."`
 	SubjectPass  struct {
 		Period time.Duration `sconf-doc:"How long unique values are accepted after generating, e.g. 12h."` // todo: have a reasonable default for this?
 	} `sconf:"optional" sconf-doc:"If configured, messages classified as weakly spam are rejected with instructions to retry delivery, but this time with a signed token added to the subject. During the next delivery attempt, the signed token will bypass the spam filter. Messages with a clear spam signal, such as a known bad reputation, are rejected/delayed without a signed token."`