When a sender writes my email address with my username uppercase, e.g. 
usern...@name.com, in the to: field of their MUA, my mail server rejects that 
email.  /var/log/mail.log prints:

   <usern...@redacted.com>: host mail.redacted.com[private/dovecot-lmtp] said: 
550 5.1.1
   >    <usern...@redacted.com> User doesn't exist: usern...@redacted.com (in 
reply to RCPT TO
   >    command)

In the manual, I found that:

   auth_username_format
   Default: %Lu
   Formatting applied to username before querying the auth database.
   You can use the standard variables here.
   Examples:
   %Lu
   Lowercases the username
   [...]

My current setting was not the default; I had:
   auth_username_format = %n

So I updated that setting to:
   auth_username_format = %Lu
expecting that %Lu would lowercase the username - and that my problem would be 
fixed.

It does not though. Updating this setting not only prompts my email server to 
still reject usern...@redacted.com, but now also rejects (lowercase username) 
usern...@redacted.com (which my server had no issue receiving beforehand).

So I reverted auth_username_format to:
   auth_username_format = %n

Emails sent to usern...@redacted.com land in my mailbox again - but emails sent 
to usern...@redacted.com still don't.

What can I try to troubleshoot?
Which config file shall I share with you?
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