diff --git a/README.md b/README.md
index f46bd9c..d1be7aa 100644
--- a/README.md
+++ b/README.md
@@ -64,6 +64,7 @@ env vars
 * **POSTMOOGLE_LOGLEVEL** - log level
 * **POSTMOOGLE_DB_DSN** - database connection string
 * **POSTMOOGLE_DB_DIALECT** - database dialect (postgres, sqlite3)
+* **POSTMOOGLE_MAILBOXES_RESERVED** - space separated list of reserved mailboxes (e.g.: `postmaster admin root`), nobody can create them
 * **POSTMOOGLE_MAXSIZE** - max email size (including attachments) in megabytes
 * **POSTMOOGLE_ADMINS** - a space-separated list of admin users. See `POSTMOOGLE_USERS` for syntax examples
 
diff --git a/bot/access.go b/bot/access.go
index cbd011b..aaca84e 100644
--- a/bot/access.go
+++ b/bot/access.go
@@ -73,6 +73,15 @@ func (b *Bot) allowSend(actorID id.UserID, targetRoomID id.RoomID) bool {
 	return !cfg.NoSend()
 }
 
+func (b *Bot) isReserved(mailbox string) bool {
+	for _, reserved := range b.reservedMailboxes {
+		if mailbox == reserved {
+			return true
+		}
+	}
+	return false
+}
+
 // IsGreylisted checks if host is in greylist
 func (b *Bot) IsGreylisted(addr net.Addr) bool {
 	if b.getBotSettings().Greylist() == 0 {
diff --git a/bot/bot.go b/bot/bot.go
index cfadf7f..c9eee4e 100644
--- a/bot/bot.go
+++ b/bot/bot.go
@@ -20,6 +20,7 @@ type Bot struct {
 	domains                 []string
 	allowedUsers            []*regexp.Regexp
 	allowedAdmins           []*regexp.Regexp
+	reservedMailboxes       []string
 	commands                commandList
 	banlist                 bglist
 	rooms                   sync.Map
@@ -36,15 +37,17 @@ func New(
 	log *logger.Logger,
 	prefix string,
 	domains []string,
+	reserved []string,
 	admins []string,
 ) (*Bot, error) {
 	b := &Bot{
-		prefix:  prefix,
-		domains: domains,
-		rooms:   sync.Map{},
-		log:     log,
-		lp:      lp,
-		mu:      map[string]*sync.Mutex{},
+		reservedMailboxes: reserved,
+		domains:           domains,
+		prefix:            prefix,
+		rooms:             sync.Map{},
+		log:               log,
+		lp:                lp,
+		mu:                map[string]*sync.Mutex{},
 	}
 	users, err := b.initBotUsers()
 	if err != nil {
diff --git a/bot/command_owner.go b/bot/command_owner.go
index c454514..a0e4970 100644
--- a/bot/command_owner.go
+++ b/bot/command_owner.go
@@ -86,7 +86,7 @@ func (b *Bot) setOption(ctx context.Context, name, value string) {
 	evt := eventFromContext(ctx)
 	if name == roomOptionMailbox {
 		existingID, ok := b.getMapping(value)
-		if ok && existingID != "" && existingID != evt.RoomID {
+		if (ok && existingID != "" && existingID != evt.RoomID) || b.isReserved(value) {
 			b.SendNotice(ctx, evt.RoomID, fmt.Sprintf("Mailbox `%s` (%s) already taken, kupo", value, utils.EmailsList(value, "")))
 			return
 		}
diff --git a/cmd/cmd.go b/cmd/cmd.go
index 7cdf3cf..60a682b 100644
--- a/cmd/cmd.go
+++ b/cmd/cmd.go
@@ -98,7 +98,7 @@ func initBot(cfg *config.Config) {
 		log.Fatal("cannot initialize matrix bot: %v", err)
 	}
 
-	mxb, err = bot.New(lp, mxlog, cfg.Prefix, cfg.Domains, cfg.Admins)
+	mxb, err = bot.New(lp, mxlog, cfg.Prefix, cfg.Domains, cfg.Mailboxes.Reserved, cfg.Admins)
 	if err != nil {
 		// nolint // Fatal = panic, not os.Exit()
 		log.Fatal("cannot start matrix bot: %v", err)
diff --git a/config/config.go b/config/config.go
index e0eeffd..63f1b04 100644
--- a/config/config.go
+++ b/config/config.go
@@ -22,6 +22,9 @@ func New() *Config {
 		MaxSize:      env.Int("maxsize", defaultConfig.MaxSize),
 		StatusMsg:    env.String("statusmsg", defaultConfig.StatusMsg),
 		Admins:       env.Slice("admins"),
+		Mailboxes: Mailboxes{
+			Reserved: env.Slice("mailboxes.reserved"),
+		},
 		TLS: TLS{
 			Certs:    env.Slice("tls.cert"),
 			Keys:     env.Slice("tls.key"),
diff --git a/config/types.go b/config/types.go
index e34c988..764212b 100644
--- a/config/types.go
+++ b/config/types.go
@@ -24,6 +24,8 @@ type Config struct {
 	MaxSize int
 	// StatusMsg of the bot
 	StatusMsg string
+	// Mailboxes config
+	Mailboxes Mailboxes
 	// Admins holds list of admin users (wildcards supported), e.g.: @*:example.com, @bot.*:example.com, @admin:*. Empty = no admins
 	Admins []string
 
@@ -57,3 +59,8 @@ type TLS struct {
 type Sentry struct {
 	DSN string
 }
+
+// Mailboxes config
+type Mailboxes struct {
+	Reserved []string
+}