Different username for passdb and userdb

[support+dovecot at shellworld.net]

Hello

I am attempting to use an external IMAP server for authentication, on 
Dovecot 2.3.16 (7e2e900c1a), running on Ubuntu 22.04.

The external server requires usernames of the format: user at domain
However, my local users, where the mail is actually drawn from, are of the 
format: user (passwd file driven).

I was able to log in with just "user", and have it both authenticate 
remotely, and access local mail, using:

passdb {
  driver = imap
  args = host=remotehost.com ssl=imaps port=993 user=%n@%d ssl_ca_dir=/etc/ssl/certs
}
userdb {
  driver = passwd
  args = blocking=no 
}

However, the remote SMTP server, which I will not be proxying, requires 
the same user at domain format for usernames. I would rather not ask 
users to use "user at domain" for SMTP, and "user" for IMAP.
They should only have to use "user at domain" for login everywhere.

Is there a way to do this? The docs suggest so 
(AuthDatabase.PasswdFile.txt), but if so I must be botching it.

Things I have tried:

passdb {
  driver = imap
  args = host=remotehost.com ssl=imaps port=993 ssl_ca_dir=/etc/ssl/certs
}
userdb {
  driver = passwd
  args = blocking=no
  override_fields = username_format=%n
}
  
I also tried:

  override_fields = user=%n

Or, alternatively to any override_fields:

  args = blocking=no username_format=%n

Lastly, I even tried this whole different approach:

userdb {
  driver = passwd-file
  args = blocking=no username_format=%n /etc/passwd
}


The following is logged no matter which of those I try:

auth: imapc(remotehost.com:993): Connected to #.#.#.#:993 (local #.#.#.#:35088)
auth: Error: passwd(user at domain,#.#.#.#,<...>): user not found from userdb
imap(274049): Error: auth-master: login: request [...]: Login auth request failed: Authenticated user not found from userdb, auth lookup id=... (auth connected 10 msecs ago, request took 1 msecs, client-pid=274038 client-id=1)
imap-login: Disconnected: Internal login failure (pid=274038 id=1): user=<user at domain>, method=PLAIN, rip=#.#.#.#, lip=#.#.#.#, mpid=274049, TLS, session=<...>

(The last attempt got a slightly varied log message, but quite similar:
auth: Error: passwd-file(user at domain#.#.#.#,<...>): user 
not found from userdb
)

Any help with this would be much appreciated.

Thanks!

Luke