There are at least two ways of finding the user name(s) being tried.
Logging SMTP transactions with Postfix
If you know which host(s) your strange connections are coming from, you can enable verbose debugging for them by specifying a debug_peer_list in /etc/postfix/main.cf:
debug_peer_list = 192.0.2.1
This will yield, among other things, a message like this in syslog:
postfix/smtpd[123]: xsasl_dovecot_handle_reply: auth reply: FAIL?5?user=info
You will want to use this setting for very specific debugging only, since the full SMTP session is logged, including the password.
postfix/smtpd[123]: < unknown[192.0.2.1]: AUTH LOGIN
postfix/smtpd[123]: xsasl_dovecot_server_first: sasl_method LOGIN
postfix/smtpd[123]: xsasl_dovecot_handle_reply: auth reply: CONT?5?VXNlcm5hbWU6
postfix/smtpd[123]: > unknown[192.0.2.1]: 334 VXNlcm5hbWU6
postfix/smtpd[123]: < unknown[192.0.2.1]: aW5mbw==
postfix/smtpd[123]: xsasl_dovecot_handle_reply: auth reply: CONT?5?UGFzc3dvcmQ6
postfix/smtpd[123]: > unknown[192.0.2.1: 334 UGFzc3dvcmQ6
postfix/smtpd[123]: < unknown[192.0.2.1]: Zm9vYmFy
postfix/smtpd[123]: xsasl_dovecot_handle_reply: auth reply: FAIL?5?user=info
postfix/smtpd[123]: warning: unknown[192.0.2.1]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
postfix/smtpd[123]: > unknown[192.0.2.1]: 535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6
In this example, a LOGIN attempt for user "info" has failed after the password "foobar" was presented. Like the challenges, you can decode the replies as base64:
$ echo aW5mbw== | echo $(base64 -d)
info
$ echo Zm9vYmFy | echo $(base64 -d)
foobar
Logging authentication with Dovecot
Postfix itself does not include a SASL implementation. Traditionally, it was hooked up to Cyrus SASL, but if you are using the Dovecot POP/IMAP server, Postfix can reuse its SASL module.
As you have found out, Dovecot has its own debugging facility, enabled with
auth_verbose = yes
in its config file, often /etc/dovecot/conf.d/10-logging.conf. The output in /var/log/dovecot-info.log will look something like this:
Jun 10 13:16:40 auth-worker(14936): Info: pam(info@example.com,192.0.2.1): pam_authenticate() failed: Authentication failure (password mismatch?)
While there is no host-based control here, there are a number of related options to control what gets logged, specifically whether or not to include the attempted passwords. From the example config:
# Log unsuccessful authentication attempts and the reasons why they failed.
auth_verbose = no
# In case of password mismatches, log the attempted password. Valid values are
# no, plain and sha1. sha1 can be useful for detecting brute force password
# attempts vs. user simply trying the same password over and over again.
auth_verbose_passwords = no
# Even more verbose logging for debugging purposes. Shows for example SQL
# queries.
auth_debug = no
# In case of password mismatches, log the passwords and used scheme so the
# problem can be debugged. Enabling this also enables auth_debug.
auth_debug_passwords = no
See also the Dovecot documentation for details.
About SMTP authentication
As you note, the string UGFzc3dvcmQ6 in the log message is the base64-encoding of "Password:". What Postfix is logging here is the particular authentication "challenge" that it has sent and received an erroneous reply for. There is also a preceding challenge for "Username:" (VXNlcm5hbWU6). However, even for nonexistent users, the failure will only be reported after the password.
The values of the challenge strings are actually unimportant and should be ignored. The first challenge is always for the user name, and the second is for the password. The details about this can be found in the specification for the LOGIN method.
NB: Since this is all a bit noodly for just transfering and checking the username-password pair, LOGIN is long obsoleted. The PLAIN method works essentially the same but packs both pieces of information into the same base64 string.
Finally, the fact that parts of this conversation are base64-encoded is actually a feature of SMTP Authentication in general. The idea is to easily allow arbitrary (possibly binary) data to be exchanged between the SASL modules of client and server, without those SASL modules needing to know or care about SMTP itself. You can get an idea of how SASL operates from the Cyrus SASL documentation.
