diff --git a/src/actor-context-api.lisp b/src/actor-context-api.lisp
index 05e2cec..19c5fe3 100644
--- a/src/actor-context-api.lisp
+++ b/src/actor-context-api.lisp
@@ -43,7 +43,7 @@ This function allows to unsubsribe from event-stream or such.
 Additional options:
 
 - `:queue-size` limits the message-box's size. By default, it is unbounded.
-- `:mbox-type` specify a custom message-box type similar as can be done is dispatcher config.
+- `:mbox-type` specify a custom message-box type similar as can be done is dispatcher config. It also could be a function which accept a `:max-queue-size` argument for `:pinned` dispatcher or `:max-queue-size` and `:dispatcher` arguments for other types of dispatchers.
   It must be a subtype of `mesgb:message-box/dp`.
 "))
 
diff --git a/src/actor-context.lisp b/src/actor-context.lisp
index 6fdd583..dd47376 100644
--- a/src/actor-context.lisp
+++ b/src/actor-context.lisp
@@ -50,22 +50,43 @@ The `actor-system` and the `actor` itself are composed of an `actor-context`."))
                                      old-actors)))))
 
 (defun %message-box-for-dispatcher-id (context dispatcher-id queue-size mbox-type)
-  (case dispatcher-id
-    (:pinned (make-instance 'mesgb:message-box/bt
-                            :max-queue-size queue-size))
-    (otherwise (let* ((asys (system context))
-                      (sys-config (asys:config asys))
-                      (disp-config (%get-dispatcher-config sys-config dispatcher-id))
-                      (dispatcher (%get-shared-dispatcher asys dispatcher-id)))
-                 (unless dispatcher
-                   (error (format nil "No such dispatcher identifier '~a' exists!" dispatcher-id)))
-                 ;; if dispatcher exists, the config does, too.
-                 (let ((eff-mbox-type (if mbox-type
-                                          mbox-type
-                                          (getf disp-config :mbox-type 'mesgb:message-box/dp))))
-                   (make-instance eff-mbox-type
-                                  :dispatcher dispatcher
-                                  :max-queue-size queue-size))))))
+  (let* ((asys (system context))
+         (sys-config (asys:config asys))
+         (disp-config (%get-dispatcher-config sys-config dispatcher-id))
+         (default-mbox-type (case dispatcher-id
+                              (:pinned 'mesgb:message-box/bt)
+                              (otherwise
+                               'mesgb:message-box/dp)))
+         (eff-mbox-type (if mbox-type
+                            mbox-type
+                            (getf disp-config :mbox-type
+                                  default-mbox-type))))
+    (case dispatcher-id
+      (:pinned
+       (etypecase eff-mbox-type
+         (function
+          (let ((result (funcall eff-mbox-type :max-queue-size queue-size)))
+            (check-type result mesgb::message-box-base)
+            result))
+         (symbol
+          (make-instance eff-mbox-type
+                         :max-queue-size queue-size))))
+      (otherwise
+       (let ((dispatcher (%get-shared-dispatcher asys dispatcher-id)))
+         (unless dispatcher
+           (error (format nil "No such dispatcher identifier '~a' exists!" dispatcher-id)))
+         ;; if dispatcher exists, the config does, too.
+         (etypecase eff-mbox-type
+           (function
+            (let ((result (funcall eff-mbox-type
+                                   :dispatcher dispatcher
+                                   :max-queue-size queue-size)))
+              (check-type result mesgb::message-box-base)
+              result))
+           (symbol
+            (make-instance eff-mbox-type
+                           :dispatcher dispatcher
+                           :max-queue-size queue-size))))))))
 
 (defun %find-actor-by-name (context name)
   (find-if (lambda (a)
diff --git a/tests/actor-context-test.lisp b/tests/actor-context-test.lisp
index aba542a..3a008fc 100644
--- a/tests/actor-context-test.lisp
+++ b/tests/actor-context-test.lisp
@@ -67,7 +67,12 @@
       (is (not (eq cut (act:context actor))))
       (is (not (null (ac:system (act:context actor))))))))
 
+
 (defclass my-mbox-type (mesgb:message-box/dp) ())
+
+(defclass my-bt-mbox-type (mesgb:message-box/bt) ())
+
+
 (test actor-of--shared--custom-mbox-type
   (with-fixture test-system ()
     (let* ((cut (make-actor-context system))
@@ -119,6 +124,57 @@
              ))
       (ac:shutdown system))))
 
+
+(test actor-of--custom-dispatcher-with-mbox-made-by-func
+  (let ((system))
+    (unwind-protect
+         (progn
+           (setf system (asys:make-actor-system `(:dispatchers
+                                                  (:foo
+                                                   (:workers 0
+                                                    :mbox-type ,(lambda (&key dispatcher max-queue-size &allow-other-keys)
+                                                                  (make-instance 'my-mbox-type
+                                                                                 :dispatcher dispatcher
+                                                                                 :max-queue-size max-queue-size)))))))
+           (let* ((cut (make-actor-context system))
+                  (actor (actor-of cut :receive (lambda ())
+                                       :dispatcher :foo)))
+             (is (not (null actor)))
+             (is (typep (act-cell:msgbox actor) 'my-mbox-type))
+             (is (eq :foo (slot-value (mesgb::dispatcher (act-cell:msgbox actor)) 'disp::identifier)))))
+      (ac:shutdown system))))
+
+
+(test actor-of--pinned-dispatcher-with-custom-mbox-type
+  (let ((system))
+    (unwind-protect
+         (progn
+           (setf system (asys:make-actor-system))
+           (let* ((cut (make-actor-context system))
+                  (actor (actor-of cut :receive (lambda ())
+                                       :dispatcher :pinned
+                                       :mbox-type 'my-bt-mbox-type)))
+             (is (not (null actor)))
+             (is (typep (act-cell:msgbox actor) 'my-bt-mbox-type))))
+      (ac:shutdown system))))
+
+
+(test actor-of--pinned-dispatcher-with-mbox-made-by-func
+  (let ((system))
+    (unwind-protect
+         (progn
+           (setf system (asys:make-actor-system))
+           (let* ((cut (make-actor-context system))
+                  (actor (actor-of cut :receive (lambda ())
+                                       :dispatcher :pinned
+                                       :mbox-type (lambda (&key max-queue-size &allow-other-keys)
+                                                    (make-instance 'my-bt-mbox-type
+                                                                   :max-queue-size max-queue-size)))))
+             (is (not (null actor)))
+             (is (typep (act-cell:msgbox actor) 'my-bt-mbox-type))))
+      (ac:shutdown system))))
+
+
 (test actor-of--err-custom-dispatcher--unknown-mbox-type
   "Tests creating an actor with a custom shared dispatcher."
   (let ((system))