diff --git a/test/tests/parallel/set3/mta/smtp_delivery.bats b/test/tests/parallel/set3/mta/smtp_delivery.bats
index 90e82f36..5e8bb263 100644
--- a/test/tests/parallel/set3/mta/smtp_delivery.bats
+++ b/test/tests/parallel/set3/mta/smtp_delivery.bats
@@ -84,13 +84,21 @@ function setup_file() {
 
   # Required for 'delivers mail to existing account':
   _send_email --data 'existing/user1'
+  assert_success
   _send_email --to user2@otherdomain.tld --data 'existing/user2'
+  assert_success
   _send_email --to user3@localhost.localdomain --data 'existing/user3'
+  assert_success
   _send_email --to added@localhost.localdomain --data 'existing/added'
+  assert_success
   _send_email --to user1@localhost.localdomain --data 'existing/user-and-cc-local-alias'
+  assert_success
   _send_email --data 'sieve/spam-folder'
+  assert_success
   _send_email --to user2@otherdomain.tld --data 'sieve/pipe'
+  assert_success
   _run_in_container_bash 'sendmail root < /tmp/docker-mailserver-test/emails/sendmail/root-email.txt'
+  assert_success
 }
 
 @test "should succeed at emptying mail queue" {
diff --git a/test/tests/serial/mail_pop3.bats b/test/tests/serial/mail_pop3.bats
index cb07484a..7a055a63 100644
--- a/test/tests/serial/mail_pop3.bats
+++ b/test/tests/serial/mail_pop3.bats
@@ -25,10 +25,12 @@ function teardown_file() { _default_teardown ; }
 
 @test 'authentication works' {
   _send_email 'auth/pop3-auth' '-w 1 0.0.0.0 110'
+  assert_success
 }
 
 @test 'added user authentication works' {
   _send_email 'auth/added-pop3-auth' '-w 1 0.0.0.0 110'
+  assert_success
 }
 
 @test '/var/log/mail/mail.log is error-free' {
diff --git a/test/tests/serial/mail_with_imap.bats b/test/tests/serial/mail_with_imap.bats
index 924e0c7f..bc648dd1 100644
--- a/test/tests/serial/mail_with_imap.bats
+++ b/test/tests/serial/mail_with_imap.bats
@@ -22,6 +22,7 @@ function teardown_file() { _default_teardown ; }
 
 @test '(Dovecot) LDAP RIMAP connection and authentication works' {
   _send_email 'auth/imap-auth' '-w 1 0.0.0.0 143'
+  assert_success
 }
 
 @test '(SASLauthd) SASL RIMAP authentication works' {
@@ -32,11 +33,14 @@ function teardown_file() { _default_teardown ; }
 @test '(SASLauthd) RIMAP SMTP authentication works' {
   _nc_wrapper 'auth/smtp-auth-login.txt' '-w 5 0.0.0.0 25'
   assert_output --partial 'Error: authentication not enabled'
+  assert_failure
 
   _nc_wrapper 'auth/smtp-auth-login.txt' '-w 5 0.0.0.0 465'
+  assert_success
   assert_output --partial 'Authentication successful'
 
   _nc_wrapper 'auth/smtp-auth-login.txt' '-w 5 0.0.0.0 587'
+  assert_success
   assert_output --partial 'Authentication successful'
 }
 
diff --git a/test/tests/serial/mail_with_ldap.bats b/test/tests/serial/mail_with_ldap.bats
index 8cab4ffc..b1c1750e 100644
--- a/test/tests/serial/mail_with_ldap.bats
+++ b/test/tests/serial/mail_with_ldap.bats
@@ -370,13 +370,16 @@ function teardown() {
   # Requires ENV `PERMIT_DOCKER=container`
   _nc_wrapper 'auth/sasl-ldap-smtp-auth.txt' '-w 5 0.0.0.0 25'
   assert_output --partial 'Error: authentication not enabled'
+  assert_failure
 
   # do not use _send_email here
   _run_in_container_bash 'openssl s_client -quiet -connect 0.0.0.0:465 < /tmp/docker-mailserver-test/auth/sasl-ldap-smtp-auth.txt'
+  assert_success
   assert_output --partial 'Authentication successful'
 
   # do not use _send_email here
   _run_in_container_bash 'openssl s_client -quiet -starttls smtp -connect 0.0.0.0:587 < /tmp/docker-mailserver-test/auth/sasl-ldap-smtp-auth.txt'
+  assert_success
   assert_output --partial 'Authentication successful'
 }
 
diff --git a/test/tests/serial/tests.bats b/test/tests/serial/tests.bats
index f3f87d10..7c6ce475 100644
--- a/test/tests/serial/tests.bats
+++ b/test/tests/serial/tests.bats
@@ -81,10 +81,12 @@ function teardown_file() { _default_teardown ; }
 
 @test "imap: authentication works" {
   _send_email 'auth/imap-auth' '-w 1 0.0.0.0 143'
+  assert_success
 }
 
 @test "imap: added user authentication works" {
   _send_email 'auth/added-imap-auth' '-w 1 0.0.0.0 143'
+  assert_success
 }
 
 #