diff --git a/setup.cfg b/setup.cfg
index d5fd84a80b8..258081301c9 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -103,6 +103,7 @@ report-timings =
     pandas==1.*
     matplotlib
 tests =
+    aiosmtpd
     async_generator
     bandit>=1.7.0
     coverage>=5.0.0,<7.3.1
diff --git a/tests/functional/events/18-workflow-event-mail.t b/tests/functional/events/18-workflow-event-mail.t
index fcde6da0bd7..0ed3b90d7d9 100755
--- a/tests/functional/events/18-workflow-event-mail.t
+++ b/tests/functional/events/18-workflow-event-mail.t
@@ -20,7 +20,7 @@
 if ! command -v mail 2>'/dev/null'; then
     skip_all '"mail" command not available'
 fi
-set_test_number 3
+set_test_number 7
 mock_smtpd_init
 OPT_SET=
 if [[ "${TEST_NAME_BASE}" == *-globalcfg ]]; then
@@ -48,13 +48,9 @@ run_ok "${TEST_NAME_BASE}-validate" \
 workflow_run_ok "${TEST_NAME_BASE}-run" \
     cylc play --reference-test --debug --no-detach ${OPT_SET} "${WORKFLOW_NAME}"
 
-contains_ok "${TEST_SMTPD_LOG}" <<__LOG__
-b'event: startup'
-b'message: workflow starting'
-b'event: shutdown'
-b'message: AUTOMATIC'
-b'see: http://localhost/stuff/${USER}/${WORKFLOW_NAME}/'
-__LOG__
+for searchterm in 'event: startup' 'message: workflow starting' 'event: shutdown' 'message: AUTOMATIC' "see: http://localhost/stuff/${USER}/${WORKFLOW_NAME}/"; do
+    grep_ok "$searchterm" "$TEST_SMTPD_LOG"
+done
 
 purge
 mock_smtpd_kill
diff --git a/tests/functional/lib/bash/test_header b/tests/functional/lib/bash/test_header
index 458a7dc908c..c4a07603126 100644
--- a/tests/functional/lib/bash/test_header
+++ b/tests/functional/lib/bash/test_header
@@ -807,10 +807,13 @@ mock_smtpd_init() {  # Logic borrowed from Rose
         local SMTPD_LOG="${TEST_DIR}/smtpd.log"
         local SMTPD_HOST="localhost:${SMTPD_PORT}"
         # Set up fake SMTP server to catch outgoing mail & redirect to log:
-        python3 -u -m 'smtpd' -c 'DebuggingServer' -d -n "${SMTPD_HOST}" \
+        python3 -u -m 'aiosmtpd' \
+            --class aiosmtpd.handlers.Debugging stdout \
+            --debug --nosetuid \
+            --listen "${SMTPD_HOST}" \
             1>"${SMTPD_LOG}" 2>&1 &  # Runs in background
         local SMTPD_PID="$!"
-        while ! grep -q 'DebuggingServer started' "${SMTPD_LOG}" 2>'/dev/null'
+        while ! grep -q 'is listening' "${SMTPD_LOG}" 2>'/dev/null'
         do
             if ps "${SMTPD_PID}" 1>/dev/null 2>&1; then
                 sleep 1  # Still waiting for fake server to start