diff --git a/test/helper/sending.bash b/test/helper/sending.bash
index 0e9013f2..ed29a98a 100644
--- a/test/helper/sending.bash
+++ b/test/helper/sending.bash
@@ -48,7 +48,7 @@ function _send_email_unchecked() {
       ( '--port' )   PORT=${2:?--port given but no argument}     ; shift 2 ;;
       ( '--data' )
         ADDITIONAL_SWAKS_OPTIONS+=('--data')
-        ADDITIONAL_SWAKS_OPTIONS+=("@/tmp/docker-mailserver-test/emails/${2:?--data given but no argument provided}.txt")
+        ADDITIONAL_SWAKS_OPTIONS+=("@/tmp/docker-mailserver-test/emails/${2:?--data given but no argument provided}")
         shift 2
         ;;
       ( * ) ADDITIONAL_SWAKS_OPTIONS+=("'${1}'") ; shift 1 ;;
diff --git a/test/tests/parallel/set1/dovecot/dovecot_quotas.bats b/test/tests/parallel/set1/dovecot/dovecot_quotas.bats
index 3b07267f..8e71bf73 100644
--- a/test/tests/parallel/set1/dovecot/dovecot_quotas.bats
+++ b/test/tests/parallel/set1/dovecot/dovecot_quotas.bats
@@ -225,9 +225,9 @@ function teardown_file() { _default_teardown ; }
   sleep 10
 
   # send some big emails
-  _send_email --to 'quotauser@otherdomain.tld' --data 'quota-exceeded'
-  _send_email --to 'quotauser@otherdomain.tld' --data 'quota-exceeded'
-  _send_email --to 'quotauser@otherdomain.tld' --data 'quota-exceeded'
+  _send_email --to 'quotauser@otherdomain.tld' --data 'quota-exceeded.txt'
+  _send_email --to 'quotauser@otherdomain.tld' --data 'quota-exceeded.txt'
+  _send_email --to 'quotauser@otherdomain.tld' --data 'quota-exceeded.txt'
   # check for quota warn message existence
   run _repeat_until_success_or_timeout 20 _exec_in_container grep -R 'Subject: quota warning' /var/mail/otherdomain.tld/quotauser/new/
   assert_success
diff --git a/test/tests/parallel/set1/dovecot/dovecot_sieve.bats b/test/tests/parallel/set1/dovecot/dovecot_sieve.bats
index 82bae8d8..6acbc4a5 100644
--- a/test/tests/parallel/set1/dovecot/dovecot_sieve.bats
+++ b/test/tests/parallel/set1/dovecot/dovecot_sieve.bats
@@ -26,9 +26,9 @@ function setup_file() {
   _wait_for_smtp_port_in_container
 
   # Single mail sent from 'spam@spam.com' that is handled by User (relocate) and Global (copy) sieves for user1:
-  _send_email --data 'sieve/spam-folder'
+  _send_email --data 'sieve/spam-folder.txt'
   # Mail for user2 triggers the sieve-pipe:
-  _send_email --to 'user2@otherdomain.tld' --data 'sieve/pipe'
+  _send_email --to 'user2@otherdomain.tld' --data 'sieve/pipe.txt'
 
   _wait_for_empty_mail_queue_in_container
 }
diff --git a/test/tests/parallel/set1/dovecot/mailbox_format_dbox.bats b/test/tests/parallel/set1/dovecot/mailbox_format_dbox.bats
index ea0af3d5..ebf0ce83 100644
--- a/test/tests/parallel/set1/dovecot/mailbox_format_dbox.bats
+++ b/test/tests/parallel/set1/dovecot/mailbox_format_dbox.bats
@@ -26,7 +26,7 @@ function teardown() { _default_teardown ; }
   _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
   _wait_for_smtp_port_in_container
 
-  _send_email --data 'existing/user1'
+  _send_email --data 'existing/user1.txt'
   _wait_for_empty_mail_queue_in_container
 
   # Mail received should be stored as `u.1` (one file per message)
@@ -47,7 +47,7 @@ function teardown() { _default_teardown ; }
   _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
   _wait_for_smtp_port_in_container
 
-  _send_email --data 'existing/user1'
+  _send_email --data 'existing/user1.txt'
   _wait_for_empty_mail_queue_in_container
 
   # Mail received should be stored in `m.1` (1 or more messages)
diff --git a/test/tests/parallel/set1/dovecot/special_use_folders.bats b/test/tests/parallel/set1/dovecot/special_use_folders.bats
index c54f949b..e6ed9715 100644
--- a/test/tests/parallel/set1/dovecot/special_use_folders.bats
+++ b/test/tests/parallel/set1/dovecot/special_use_folders.bats
@@ -14,7 +14,7 @@ function setup_file() {
 function teardown_file() { _default_teardown ; }
 
 @test 'normal delivery works' {
-  _send_email --data 'existing/user1'
+  _send_email --data 'existing/user1.txt'
   _count_files_in_directory_in_container /var/mail/localhost.localdomain/user1/new 1
 }
 
diff --git a/test/tests/parallel/set1/spam_virus/clamav.bats b/test/tests/parallel/set1/spam_virus/clamav.bats
index 4c8e6ec9..15eec0c1 100644
--- a/test/tests/parallel/set1/spam_virus/clamav.bats
+++ b/test/tests/parallel/set1/spam_virus/clamav.bats
@@ -25,7 +25,7 @@ function setup_file() {
 
   _wait_for_service postfix
   _wait_for_smtp_port_in_container
-  _send_email --from 'virus@external.tld' --data 'amavis/virus'
+  _send_email --from 'virus@external.tld' --data 'amavis/virus.txt'
   _wait_for_empty_mail_queue_in_container
 }
 
diff --git a/test/tests/parallel/set1/spam_virus/disabled_clamav_spamassassin.bats b/test/tests/parallel/set1/spam_virus/disabled_clamav_spamassassin.bats
index d6205b10..4cae4e7a 100644
--- a/test/tests/parallel/set1/spam_virus/disabled_clamav_spamassassin.bats
+++ b/test/tests/parallel/set1/spam_virus/disabled_clamav_spamassassin.bats
@@ -18,7 +18,7 @@ function setup_file() {
   _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
   _wait_for_smtp_port_in_container
 
-  _send_email --data 'existing/user1'
+  _send_email --data 'existing/user1.txt'
   _wait_for_empty_mail_queue_in_container
 }
 
diff --git a/test/tests/parallel/set1/spam_virus/postgrey_enabled.bats b/test/tests/parallel/set1/spam_virus/postgrey_enabled.bats
index 238e0d37..188308df 100644
--- a/test/tests/parallel/set1/spam_virus/postgrey_enabled.bats
+++ b/test/tests/parallel/set1/spam_virus/postgrey_enabled.bats
@@ -51,7 +51,7 @@ function teardown_file() { _default_teardown ; }
   _reload_postfix
 
   # Send test mail (it should fail to deliver):
-  _send_email_unchecked --from 'user@external.tld' --port 25 --data 'postgrey'
+  _send_email_unchecked --from 'user@external.tld' --port 25 --data 'postgrey.txt'
   assert_failure
   assert_output --partial 'Recipient address rejected: Delayed by Postgrey'
 
@@ -67,7 +67,7 @@ function teardown_file() { _default_teardown ; }
   # Wait until `$POSTGREY_DELAY` seconds pass before trying again:
   sleep 3
   # Retry delivering test mail (it should be trusted this time):
-  _send_email --from 'user@external.tld' --port 25 --data 'postgrey'
+  _send_email --from 'user@external.tld' --port 25 --data 'postgrey.txt'
 
   # Confirm postgrey permitted delivery (triplet is now trusted):
   _should_have_log_entry \
diff --git a/test/tests/parallel/set1/spam_virus/postscreen.bats b/test/tests/parallel/set1/spam_virus/postscreen.bats
index 75b14264..493eb228 100644
--- a/test/tests/parallel/set1/spam_virus/postscreen.bats
+++ b/test/tests/parallel/set1/spam_virus/postscreen.bats
@@ -57,8 +57,8 @@ function teardown_file() {
   # Configure `send_email()` to send from the mail client container (CONTAINER2_NAME) via ENV override,
   # mail is sent to the DMS server container (CONTAINER1_NAME) via `--server` parameter:
   # TODO: Use _send_email_and_get_id when proper resolution of domain names is possible:
-  CONTAINER_NAME=${CONTAINER2_NAME} _send_email_unchecked --server "${CONTAINER1_IP}" --port 25 --data 'postscreen'
-  # CONTAINER_NAME=${CONTAINER2_NAME} _send_email_and_get_id MAIL_ID_POSTSCREEN --server "${CONTAINER1_IP}" --data 'postscreen'
+  CONTAINER_NAME=${CONTAINER2_NAME} _send_email_unchecked --server "${CONTAINER1_IP}" --port 25 --data 'postscreen.txt'
+  # CONTAINER_NAME=${CONTAINER2_NAME} _send_email_and_get_id MAIL_ID_POSTSCREEN --server "${CONTAINER1_IP}" --data 'postscreen.txt'
   # _print_mail_log_for_id "${MAIL_ID_POSTSCREEN}"
   # assert_output --partial "stored mail into mailbox 'INBOX'"
 
diff --git a/test/tests/parallel/set1/spam_virus/spam_junk_folder.bats b/test/tests/parallel/set1/spam_virus/spam_junk_folder.bats
index fea23b0b..2c3a522d 100644
--- a/test/tests/parallel/set1/spam_virus/spam_junk_folder.bats
+++ b/test/tests/parallel/set1/spam_virus/spam_junk_folder.bats
@@ -95,7 +95,7 @@ function teardown() { _default_teardown ; }
 function _should_send_spam_message() {
   _wait_for_smtp_port_in_container
   _wait_for_tcp_port_in_container 10024 # port 10024 is for Amavis
-  _send_email --from 'spam@external.tld' --data 'amavis/spam'
+  _send_email --from 'spam@external.tld' --data 'amavis/spam.txt'
 }
 
 function _should_be_received_by_amavis() {
diff --git a/test/tests/parallel/set3/container_configuration/hostname.bats b/test/tests/parallel/set3/container_configuration/hostname.bats
index f5774eef..a262b2e7 100644
--- a/test/tests/parallel/set3/container_configuration/hostname.bats
+++ b/test/tests/parallel/set3/container_configuration/hostname.bats
@@ -207,7 +207,7 @@ function _should_have_correct_mail_headers() {
   # (eg: OVERRIDE_HOSTNAME or `--hostname mail --domainname example.test`)
   local EXPECTED_HOSTNAME=${3:-${EXPECTED_FQDN}}
 
-  _send_email --from 'user@external.tld' --data 'existing/user1'
+  _send_email --from 'user@external.tld' --data 'existing/user1.txt'
   _wait_for_empty_mail_queue_in_container
   _count_files_in_directory_in_container '/var/mail/localhost.localdomain/user1/new/' '1'
 
diff --git a/test/tests/parallel/set3/mta/lmtp_ip.bats b/test/tests/parallel/set3/mta/lmtp_ip.bats
index d8be42d9..f86c7538 100644
--- a/test/tests/parallel/set3/mta/lmtp_ip.bats
+++ b/test/tests/parallel/set3/mta/lmtp_ip.bats
@@ -38,7 +38,7 @@ function teardown_file() { _default_teardown ; }
 
 @test "delivers mail to existing account" {
   _wait_for_smtp_port_in_container
-  _send_email --data 'existing/user1' # send a test email
+  _send_email --data 'existing/user1.txt' # send a test email
 
   # Verify delivery was successful, log line should look similar to:
   # postfix/lmtp[1274]: 0EA424ABE7D9: to=<user1@localhost.localdomain>, relay=127.0.0.1[127.0.0.1]:24, delay=0.13, delays=0.07/0.01/0.01/0.05, dsn=2.0.0, status=sent (250 2.0.0 <user1@localhost.localdomain> ixPpB+Zvv2P7BAAAUi6ngw Saved)
diff --git a/test/tests/parallel/set3/mta/privacy.bats b/test/tests/parallel/set3/mta/privacy.bats
index 55358224..614e2e87 100644
--- a/test/tests/parallel/set3/mta/privacy.bats
+++ b/test/tests/parallel/set3/mta/privacy.bats
@@ -29,7 +29,7 @@ function teardown_file() { _default_teardown ; }
     --port 587 -tls --auth PLAIN \
     --auth-user user1@localhost.localdomain \
     --auth-password mypassword \
-    --data 'privacy'
+    --data 'privacy.txt'
 
   _run_until_success_or_timeout 120 _exec_in_container_bash '[[ -d /var/mail/localhost.localdomain/user1/new ]]'
   assert_success
diff --git a/test/tests/parallel/set3/mta/smtp_delivery.bats b/test/tests/parallel/set3/mta/smtp_delivery.bats
index 5ec19e72..e0926fce 100644
--- a/test/tests/parallel/set3/mta/smtp_delivery.bats
+++ b/test/tests/parallel/set3/mta/smtp_delivery.bats
@@ -63,34 +63,34 @@ function setup_file() {
 
   # TODO: Move to clamav tests (For use when ClamAV is enabled):
   # _repeat_in_container_until_success_or_timeout 60 "${CONTAINER_NAME}" test -e /var/run/clamav/clamd.ctl
-  # _send_email --from 'virus@external.tld' --data 'amavis/virus'
+  # _send_email --from 'virus@external.tld' --data 'amavis/virus.txt'
 
   # Required for 'delivers mail to existing alias':
-  _send_email --to alias1@localhost.localdomain --data 'existing/alias-external'
+  _send_email --to alias1@localhost.localdomain --data 'existing/alias-external.txt'
   # Required for 'delivers mail to existing alias with recipient delimiter':
-  _send_email --to alias1~test@localhost.localdomain --data 'existing/alias-recipient-delimiter'
+  _send_email --to alias1~test@localhost.localdomain --data 'existing/alias-recipient-delimiter.txt'
   # Required for 'delivers mail to existing catchall':
-  _send_email --to wildcard@localdomain2.com --data 'existing/catchall-local'
+  _send_email --to wildcard@localdomain2.com --data 'existing/catchall-local.txt'
   # Required for 'delivers mail to regexp alias':
-  _send_email --to test123@localhost.localdomain --data 'existing/regexp-alias-local'
+  _send_email --to test123@localhost.localdomain --data 'existing/regexp-alias-local.txt'
 
   # Required for 'rejects mail to unknown user':
-  _send_email_unchecked --to nouser@localhost.localdomain --data 'non-existing-user'
+  _send_email_unchecked --to nouser@localhost.localdomain --data 'non-existing-user.txt'
   assert_failure
   # Required for 'redirects mail to external aliases':
-  _send_email --to bounce-always@localhost.localdomain --data 'existing/regexp-alias-external'
-  _send_email --to alias2@localhost.localdomain --data 'existing/alias-local'
+  _send_email --to bounce-always@localhost.localdomain --data 'existing/regexp-alias-external.txt'
+  _send_email --to alias2@localhost.localdomain --data 'existing/alias-local.txt'
   # Required for 'rejects spam':
-  _send_email --from 'spam@external.tld' --data 'amavis/spam'
+  _send_email --from 'spam@external.tld' --data 'amavis/spam.txt'
 
   # Required for 'delivers mail to existing account':
-  _send_email --data 'existing/user1'
+  _send_email --data 'existing/user1.txt'
   _send_email --to user2@otherdomain.tld
   _send_email --to user3@localhost.localdomain
-  _send_email --to added@localhost.localdomain --data 'existing/added'
-  _send_email --to user1@localhost.localdomain --data 'existing/user-and-cc-local-alias'
-  _send_email --data 'sieve/spam-folder'
-  _send_email --to user2@otherdomain.tld --data 'sieve/pipe'
+  _send_email --to added@localhost.localdomain --data 'existing/added.txt'
+  _send_email --to user1@localhost.localdomain --data 'existing/user-and-cc-local-alias.txt'
+  _send_email --data 'sieve/spam-folder.txt'
+  _send_email --to user2@otherdomain.tld --data 'sieve/pipe.txt'
   _run_in_container_bash 'sendmail root < /tmp/docker-mailserver-test/emails/sendmail/root-email.txt'
   assert_success
 }
diff --git a/test/tests/serial/mail_with_ldap.bats b/test/tests/serial/mail_with_ldap.bats
index b561b654..76a176b5 100644
--- a/test/tests/serial/mail_with_ldap.bats
+++ b/test/tests/serial/mail_with_ldap.bats
@@ -332,7 +332,7 @@ function teardown() {
     --auth-password secret \
     --ehlo mail \
     --from ldap@localhost.localdomain \
-    --data 'auth/ldap-smtp-auth-spoofed'
+    --data 'auth/ldap-smtp-auth-spoofed.txt'
   assert_failure
   assert_output --partial 'Sender address rejected: not owned by user'
 }
@@ -345,7 +345,7 @@ function teardown() {
     --ehlo mail \
     --from postmaster@localhost.localdomain \
     --to some.user@localhost.localdomain \
-    --data 'auth/ldap-smtp-auth-spoofed-alias'
+    --data 'auth/ldap-smtp-auth-spoofed-alias.txt'
   assert_output --partial 'End data with'
 }
 
@@ -361,7 +361,7 @@ function teardown() {
     --ehlo mail \
     --from randomspoofedaddress@localhost.localdomain \
     --to some.user@localhost.localdomain \
-    --data 'auth/ldap-smtp-auth-spoofed-sender-with-filter-exception'
+    --data 'auth/ldap-smtp-auth-spoofed-sender-with-filter-exception.txt'
   assert_failure
   assert_output --partial 'Sender address rejected: not owned by user'
 }
diff --git a/test/tests/serial/tests.bats b/test/tests/serial/tests.bats
index 4537bdcf..a9168546 100644
--- a/test/tests/serial/tests.bats
+++ b/test/tests/serial/tests.bats
@@ -299,7 +299,7 @@ EOF
     --auth-password mypassword \
     --ehlo mail \
     --from user2@localhost.localdomain \
-    --data 'auth/added-smtp-auth-spoofed'
+    --data 'auth/added-smtp-auth-spoofed.txt'
   assert_output --partial 'Sender address rejected: not owned by user'
 }
 
@@ -315,7 +315,7 @@ EOF
     --auth-password mypassword \
     --ehlo mail \
     --from alias1@localhost.localdomain \
-    --data 'auth/added-smtp-auth-spoofed-alias'
+    --data 'auth/added-smtp-auth-spoofed-alias.txt'
   assert_success
   assert_output --partial 'End data with'
 }
diff --git a/test/tests/serial/vmail-id.bats b/test/tests/serial/vmail-id.bats
index 0f54ea96..c348f177 100644
--- a/test/tests/serial/vmail-id.bats
+++ b/test/tests/serial/vmail-id.bats
@@ -20,7 +20,7 @@ function setup_file() {
 function teardown_file() { _default_teardown ; }
 
 @test 'should successfully deliver mail' {
-  _send_email --data 'existing/user1'
+  _send_email --data 'existing/user1.txt'
   _wait_for_empty_mail_queue_in_container
 
   # Should be successfully sent (received) by Postfix: