diff --git a/rel/mim1.vars-toml.config b/rel/mim1.vars-toml.config
index 6af5aba5bc2..0aa1b9467e1 100644
--- a/rel/mim1.vars-toml.config
+++ b/rel/mim1.vars-toml.config
@@ -68,6 +68,7 @@
   shaper = \"c2s_shaper\"
   max_stanza_size = 65536
   tls.certfile = \"priv/ssl/fake_server.pem\"
+  tls.cacertfile = \"priv/ssl/cacert.pem\"
   tls.mode = \"tls\""}.
 {listen_service,
   "[[listen.service]]
diff --git a/src/config/mongoose_config_spec.erl b/src/config/mongoose_config_spec.erl
index 943a3635d5e..3843d273044 100644
--- a/src/config/mongoose_config_spec.erl
+++ b/src/config/mongoose_config_spec.erl
@@ -9,12 +9,14 @@
          tls/2]).
 
 %% callbacks for the 'process' step
--export([process_root/1,
+-export([process_dynamic_domains/1,
+         process_s2s/1,
          process_host/1,
          process_general/1,
          process_listener/2,
          process_c2s_tls/1,
          process_c2s_just_tls/1,
+         process_c2s_fast_tls/1,
          process_just_tls/1,
          process_fast_tls/1,
          process_sasl_external/1,
@@ -106,7 +108,7 @@ root() ->
                 },
        defaults = #{<<"internal_databases">> => default_internal_databases()},
        required = [<<"general">>],
-       process = fun ?MODULE:process_root/1,
+       process = [fun ?MODULE:process_dynamic_domains/1, fun ?MODULE:process_s2s/1],
        wrap = none,
        format_items = list
       }.
@@ -701,7 +703,7 @@ tls(c2s, just_tls) ->
                                                                 validate = filename}}},
              process = fun ?MODULE:process_c2s_just_tls/1};
 tls(c2s, fast_tls) ->
-    #section{}.
+    #section{process = fun ?MODULE:process_c2s_fast_tls/1}.
 
 server_name_indication() ->
     #section{items = #{<<"enabled">> => #option{type = boolean},
@@ -938,8 +940,9 @@ s2s_address() ->
 
 %% Callbacks for 'process'
 
+%% Callback for root level `process` function
 %% Check that all auth methods and modules enabled for any host type support dynamic domains
-process_root(Items) ->
+process_dynamic_domains(Items) ->
     case proplists:lookup(host_types, Items) of
         {_, [_|_] = HostTypes} ->
             HTItems = lists:filter(fun(Item) -> is_host_type_item(Item, HostTypes) end, Items),
@@ -975,6 +978,48 @@ extract_modules(KVs) ->
                                  (_) -> []
                               end, KVs)).
 
+%% Callback for root level `process` function
+%% Ensure that CA Certificate file (`cacertfile` option) is provided for s2s listeners
+%% if user configured s2s `use_starttls` as `required` or `required_trusted`
+%% for at least one host, host_type or globally.
+process_s2s(Items) ->
+    UseStartTlsValues = lists:flatmap(fun({{s2s, _}, S2S}) -> [maps:get(use_starttls, S2S)];
+                                          (_) -> [] end, Items),
+    case lists:any(fun is_starttls_required/1, UseStartTlsValues) of
+        true ->
+            check_s2s_verify_mode_cacertfile(Items);
+        _ ->
+            Items
+    end.
+
+is_starttls_required(required) ->
+    true;
+is_starttls_required(required_trusted) ->
+    true;
+is_starttls_required(_) ->
+    false.
+
+check_s2s_verify_mode_cacertfile(Items) ->
+    case lists:keyfind(listen, 1, Items) of
+        false ->
+            Items;
+        {_, ListenItems} ->
+            S2S = [Item || Item <- ListenItems, maps:get(module, Item) == ejabberd_s2s_in],
+            lists:foreach(fun verify_s2s_tls/1, S2S),
+            Items
+    end.
+
+verify_s2s_tls(#{tls := #{cacertfile := _}} = Item) ->
+    Item;
+verify_s2s_tls(#{tls := #{verify_mode := none}} = Item) ->
+    Item;
+verify_s2s_tls(_) ->
+    error(#{what => missing_cacertfile,
+            text => <<"You need to provide CA certificate (cacertfile) "
+                       "or disable peer verification (set `verify_mode` to `none`) "
+                       "in the `s2s.listen` sections when any of the s2s sections "
+                       "contains use_starttls as `required` or `required_trusted`.">>}).
+
 is_host_type_item({{_, HostType}, _}, HostTypes) ->
     HostType =:= global orelse lists:member(HostType, HostTypes);
 is_host_type_item(_, _) ->
@@ -1045,6 +1090,17 @@ fast_tls_defaults() ->
     #{ciphers => mongoose_tls:default_ciphers(),
       protocol_options => ["no_sslv2", "no_sslv3", "no_tlsv1", "no_tlsv1_1"]}.
 
+process_c2s_fast_tls(M = #{module := just_tls}) ->
+    M;
+process_c2s_fast_tls(M = #{cacertfile := _}) ->
+    M;
+process_c2s_fast_tls(M = #{verify_mode := none}) ->
+    M;
+process_c2s_fast_tls(_) ->
+    error(#{what => missing_cacertfile,
+            text => <<"You need to provide CA certificate (cacertfile) "
+                      "or disable peer verification (verify_mode)">>}).
+
 process_listener([item, Type | _], Opts) ->
     mongoose_listener_config:ensure_ip_options(Opts#{module => listener_module(Type),
                                                      connection_type => connection_type(Type)}).
diff --git a/test/common/config_parser_helper.erl b/test/common/config_parser_helper.erl
index 6d101e949cd..9d78f925b0f 100644
--- a/test/common/config_parser_helper.erl
+++ b/test/common/config_parser_helper.erl
@@ -156,7 +156,8 @@ options("mongooseim-pgsql") ->
                 access => c2s,
                 shaper => c2s_shaper,
                 max_stanza_size => 65536,
-                tls => #{certfile => "priv/dc1.pem", dhfile => "priv/dh.pem"}
+                tls => #{certfile => "priv/dc1.pem", dhfile => "priv/dh.pem",
+                         cacertfile => "priv/ca.pem"}
                }),
        config([listen, c2s],
               #{port => 5223,
diff --git a/test/config_parser_SUITE.erl b/test/config_parser_SUITE.erl
index d0de5346008..90c6ce5eaa2 100644
--- a/test/config_parser_SUITE.erl
+++ b/test/config_parser_SUITE.erl
@@ -94,6 +94,7 @@ groups() ->
                            listen_c2s_just_tls,
                            listen_s2s,
                            listen_s2s_tls,
+                           listen_s2s_cacertfile_verify,
                            listen_service,
                            listen_http,
                            listen_http_tls,
@@ -508,11 +509,18 @@ listen_c2s_fast_tls(_Config) ->
     T = fun(Opts) -> listen_raw(c2s, #{<<"port">> => 5222,
                                        <<"tls">> => Opts}) end,
     P = [listen, 1, tls],
-    ?cfg(P, default_c2s_tls(fast_tls), T(#{})),
+    M = tls_ca_raw(),
+    ?cfg(P, maps:merge(default_c2s_tls(fast_tls), tls_ca()), T(M)),
     test_fast_tls_server(P, T),
-    ?cfg(P ++ [mode], tls, T(#{<<"mode">> => <<"tls">>})),
-    ?err(T(#{<<"mode">> => <<"stopttls">>})),
-    ?err(T(#{<<"module">> => <<"slow_tls">>})).
+    %% we do not require `cacertfile` when `verify_mode` is `none`
+    ?cfg(P ++ [verify_mode], none, T(#{<<"verify_mode">> => <<"none">>})),
+    %% we require `cacertfile` when `verify_mode` is `peer` (which is the default)
+    ?cfg(P ++ [cacertfile], "priv/ca.pem", T(M#{<<"verify_mode">> => <<"peer">>})),
+    ?err([#{reason := missing_cacertfile}], T(#{})),
+    ?err([#{reason := missing_cacertfile}], T(#{<<"verify_mode">> => <<"peer">>})),
+    ?cfg(P ++ [mode], tls, T(M#{<<"mode">> => <<"tls">>})),
+    ?err(T(M#{<<"mode">> => <<"stopttls">>})),
+    ?err(T(M#{<<"module">> => <<"slow_tls">>})).
 
 listen_c2s_just_tls(_Config) ->
     T = fun(Opts) -> listen_raw(c2s, #{<<"port">> => 5222,
@@ -545,6 +553,28 @@ listen_s2s_tls(_Config) ->
     ?cfg(P, default_config([listen, s2s, tls]), T(#{})),
     test_fast_tls_server(P, T).
 
+listen_s2s_cacertfile_verify(_Config) ->
+    T = fun(UseStartTLS, Opts) ->
+            maps:merge(#{<<"s2s">> => #{<<"use_starttls">> => UseStartTLS}},
+            listen_raw(s2s, #{<<"port">> => 5269, <<"tls">> => Opts})) end,
+    P = [listen, 1, tls],
+    ConfigWithCA = maps:merge(default_config([listen, s2s, tls]), tls_ca()),
+    %% no checking of `cacertfile` when `use_starttls` is `false` or `optional`
+    ?cfg(P, default_config([listen, s2s, tls]), T(<<"false">>, #{})),
+    ?cfg(P, default_config([listen, s2s, tls]), T(<<"optional">>, #{})),
+    %% `cacertfile` is required when `use_starttls` is `required` or `optional`
+    ?cfg(P, ConfigWithCA, T(<<"required">>, tls_ca_raw())),
+    ?cfg(P, ConfigWithCA, T(<<"required_trusted">>, tls_ca_raw())),
+    ?err([#{reason := missing_cacertfile}], T(<<"required">>, #{})),
+    ?err([#{reason := missing_cacertfile}], T(<<"required_trusted">>, #{})),
+    %% setting `verify_mode` to `none` turns off `cacertfile` validation
+    VerifyModeNone = #{verify_mode => none},
+    VerifyModeNoneRaw = #{<<"verify_mode">> => <<"none">>},
+    ConfigWithVerifyModeNone = maps:merge(default_config([listen, s2s, tls]),
+                                          #{verify_mode => none}),
+    ?cfg(P, ConfigWithVerifyModeNone, T(<<"required">>, VerifyModeNoneRaw)),
+    ?cfg(P, ConfigWithVerifyModeNone, T(<<"required_trusted">>, VerifyModeNoneRaw)).
+
 listen_service(_Config) ->
     T = fun(Opts) -> listen_raw(service, maps:merge(#{<<"port">> => 8888,
                                                       <<"password">> => <<"secret">>}, Opts))
@@ -1197,12 +1227,13 @@ test_just_tls_client_sni(ParentP, ParentT) ->
 
 test_fast_tls_server(P, T) ->
     ?cfg(P ++ [verify_mode], none, T(#{<<"verify_mode">> => <<"none">>})),
-    ?cfg(P ++ [certfile], "priv/cert.pem", T(#{<<"certfile">> => <<"priv/cert.pem">>})),
-    ?cfg(P ++ [cacertfile], "priv/ca.pem", T(tls_ca_raw())),
+    M = tls_ca_raw(),
+    ?cfg(P ++ [certfile], "priv/cert.pem", T(M#{<<"certfile">> => <<"priv/cert.pem">>})),
+    ?cfg(P ++ [cacertfile], "priv/ca.pem", T(M)),
     ?cfg(P ++ [ciphers], "TLS_AES_256_GCM_SHA384",
-         T(#{<<"ciphers">> => <<"TLS_AES_256_GCM_SHA384">>})),
-    ?cfg(P ++ [dhfile], "priv/dh.pem", T(#{<<"dhfile">> => <<"priv/dh.pem">>})),
-    ?cfg(P ++ [protocol_options], ["nosslv2"], T(#{<<"protocol_options">> => [<<"nosslv2">>]})),
+         T(M#{<<"ciphers">> => <<"TLS_AES_256_GCM_SHA384">>})),
+    ?cfg(P ++ [dhfile], "priv/dh.pem", T(M#{<<"dhfile">> => <<"priv/dh.pem">>})),
+    ?cfg(P ++ [protocol_options], ["nosslv2"], T(M#{<<"protocol_options">> => [<<"nosslv2">>]})),
     ?err(T(#{<<"verify_mode">> => <<"selfsigned_peer">>})), % value only for just_tls
     ?err(T(#{<<"crl_files">> => [<<"priv/cert.pem">>]})), % option only for just_tls
     ?err(T(#{<<"certfile">> => <<"no_such_file.pem">>})),
diff --git a/test/config_parser_SUITE_data/mongooseim-pgsql.toml b/test/config_parser_SUITE_data/mongooseim-pgsql.toml
index e8ed093fe62..19b33248e11 100644
--- a/test/config_parser_SUITE_data/mongooseim-pgsql.toml
+++ b/test/config_parser_SUITE_data/mongooseim-pgsql.toml
@@ -86,6 +86,7 @@
   tls.mode = "starttls"
   tls.certfile = "priv/dc1.pem"
   tls.dhfile = "priv/dh.pem"
+  tls.cacertfile = "priv/ca.pem"
 
 [[listen.c2s]]
   port = 5223