diff --git a/2023.12/apache/setup_msmtp.sh b/2023.12/apache/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2023.12/apache/setup_msmtp.sh
+++ b/2023.12/apache/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2023.12/fpm-alpine/setup_msmtp.sh b/2023.12/fpm-alpine/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2023.12/fpm-alpine/setup_msmtp.sh
+++ b/2023.12/fpm-alpine/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2023.12/fpm/setup_msmtp.sh b/2023.12/fpm/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2023.12/fpm/setup_msmtp.sh
+++ b/2023.12/fpm/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2024.03-dev/apache/setup_msmtp.sh b/2024.03-dev/apache/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2024.03-dev/apache/setup_msmtp.sh
+++ b/2024.03-dev/apache/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2024.03-dev/fpm-alpine/setup_msmtp.sh b/2024.03-dev/fpm-alpine/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2024.03-dev/fpm-alpine/setup_msmtp.sh
+++ b/2024.03-dev/fpm-alpine/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2024.03-dev/fpm/setup_msmtp.sh b/2024.03-dev/fpm/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2024.03-dev/fpm/setup_msmtp.sh
+++ b/2024.03-dev/fpm/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2024.03-rc/apache/setup_msmtp.sh b/2024.03-rc/apache/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2024.03-rc/apache/setup_msmtp.sh
+++ b/2024.03-rc/apache/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2024.03-rc/fpm-alpine/setup_msmtp.sh b/2024.03-rc/fpm-alpine/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2024.03-rc/fpm-alpine/setup_msmtp.sh
+++ b/2024.03-rc/fpm-alpine/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/2024.03-rc/fpm/setup_msmtp.sh b/2024.03-rc/fpm/setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/2024.03-rc/fpm/setup_msmtp.sh
+++ b/2024.03-rc/fpm/setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi
diff --git a/README.md b/README.md
index 5c5a439..b492f41 100644
--- a/README.md
+++ b/README.md
@@ -123,11 +123,16 @@ The following environment variables are possible for the SMTP examples.
 -	`SMTP_DOMAIN` The sender domain. (**required** - e.g. `friendica.local`)
 -	`SMTP_FROM` Sender user-part of the address. (Default: `no-reply` - e.g. no-reply@friendica.local)
 -	`SMTP_TLS` Use TLS for connecting the SMTP Mail-Gateway. (Default: empty)
--	`SMTP_STARTTLS` Use STARTTLS for connecting the SMTP Mail-Gateway. (Default: empty)
+-	`SMTP_STARTTLS` Use STARTTLS for connecting the SMTP Mail-Gateway. (Default: `On`)
 -	`SMTP_AUTH` Auth mode for the SMTP Mail-Gateway. (Default: `On`)
 -	`SMTP_AUTH_USER` Username for the SMTP Mail-Gateway. (Default: empty)
 -	`SMTP_AUTH_PASS` Password for the SMTP Mail-Gateway. (Default: empty)
 
+**Addition to STARTTLS**
+
+the `tls_starttls` setting is either `On` or `Off`, but never unset.
+That's because in case it's unset, `starttls` would be activated by default (which would need additional configuration like a separate port).
+
 ## Database settings
 
 You have to add the Friendica container to the same network as the running database container, e. g. `--network some-network`, and then use `mysql` as the database host on setup.
diff --git a/docker-setup_msmtp.sh b/docker-setup_msmtp.sh
index 654883c..c902b6d 100644
--- a/docker-setup_msmtp.sh
+++ b/docker-setup_msmtp.sh
@@ -7,6 +7,8 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
 
   smtp_from="${SMTP_FROM:=no-reply}"
   smtp_auth="${SMTP_AUTH:=on}"
+  # https://github.com/friendica/docker/issues/233
+  smtp_starttls="${SMTP_STARTTLS:=on}"
 
   # Setup MSMTP
   usermod --comment "$(echo "$SITENAME" | tr -dc '[:print:]')" root
@@ -26,7 +28,7 @@ if [ -n "${SMTP_DOMAIN+x}" ] && [ -n "${SMTP+x}" ] && [ "${SMTP}" != "localhost"
     echo "from \"$smtp_from@$SMTP_DOMAIN\""
     echo "tls_certcheck off" # No certcheck because of internal docker mail-hostnames
     if [ -n "${SMTP_TLS+x}" ]; then echo "tls on"; fi
-    if [ -n "${SMTP_STARTTLS+x}" ]; then echo "tls_starttls on"; fi
+    echo "tls_starttls $smtp_starttls";
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "auth $smtp_auth"; fi
     if [ -n "${SMTP_AUTH_USER+x}" ]; then echo "user \"$SMTP_AUTH_USER\""; fi
     if [ -n "${SMTP_AUTH_PASS+x}" ]; then echo "password \"$SMTP_AUTH_PASS\""; fi