When a sender writes my email address with my username uppercase, e.g. USERNAME@name.com, in the to: field of their MUA, my mail server rejects that email. /var/log/mail.log prints:
<USERNAME@redacted.com>: host mail.redacted.com[private/dovecot-lmtp] said: 550 5.1.1
<USERNAME@redacted.com> User doesn't exist: USERNAME@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 USERNAME@redacted.com, but now also rejects (lowercase username) username@redacted.com (which my server had no issue receiving beforehand).
So I reverted auth_username_format to: auth_username_format = %n
Emails sent to username@redacted.com land in my mailbox again - but emails sent to USERNAME@redacted.com still don't.
What can I try to troubleshoot? Which config file shall I share with you?