Constant slslauthd failures, not being blocked?

SYSTEM INFORMATION
OS type and version: Debian Linux 10.11
Webmin version: 1.981
Virtualmin version: 6.17-3 Pro
Related products version: Postfix v3.4.14, Fail2Ban v0.10.2

I am seeing a constant (about one every minute or two) entries in /var/log/auth.log for failed smtp connection attempts, with no IP address listed:

Dec 23 11:43:59 mondo saslauthd[650]: pam_unix(smtp:auth): check pass; user unknown
Dec 23 11:43:59 mondo saslauthd[650]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Dec 23 11:44:01 mondo saslauthd[650]: DEBUG: auth_pam: pam_authenticate failed: Authentication failure
Dec 23 11:44:01 mondo saslauthd[650]:                 : auth failure: [user=postmaster] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]
Dec 23 11:44:27 mondo saslauthd[653]: pam_unix(smtp:auth): check pass; user unknown
Dec 23 11:44:27 mondo saslauthd[653]: pam_unix(smtp:auth): authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
Dec 23 11:44:29 mondo saslauthd[653]: DEBUG: auth_pam: pam_authenticate failed: Authentication failure
Dec 23 11:44:29 mondo saslauthd[653]:                 : auth failure: [user=reception] [service=smtp] [realm=] [mech=pam] [reason=PAM auth error]

Fail2Ban has the postfix-sasl jail enabled and everything there is default. But it does not seem to be picking the above entries up and blocking them:

fail2ban-client status postfix-sasl
Status for the jail: postfix-sasl
|- Filter
|  |- Currently failed: 5
|  |- Total failed:     43
|  `- File list:        /var/log/mail.log
`- Actions
   |- Currently banned: 0
   |- Total banned:     0
   `- Banned IP list:

Any suggestions on what I should check?

Post-holiday bump . . .