Comment 9 for bug 1940603

I finally got around to take another look, sorry for the delay. The problem is the "noplaintext" in smtpd_sasl_security_options Here's a small config diff that fixes the problem:

# diff -Naur main.cf.bug main.cf
--- main.cf.bug 2021-09-15 19:14:02.919982259 +0000
+++ main.cf 2021-09-15 19:18:04.765338947 +0000
@@ -48,8 +48,7 @@
 smtpd_sasl_type = dovecot
 smtpd_sasl_path = private/auth
 smtpd_sasl_local_domain =
-smtpd_sasl_security_options = noanonymous,noplaintext
-smtpd_sasl_tls_security_options = noanonymous
+smtpd_tls_auth_only = yes
 broken_sasl_auth_clients = yes
 smtpd_sasl_auth_enable = yes
 smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination

I believe this confirms the problem to be a local config one so I'll mark the bug as invalid. Let me know if the above diff doesn't make it work for you.