Findings:

options.mail_password in the DB contains the plaintext 48-char value of CLOUDRON_MAIL_SMTP_PASSWORD instead of a Laravel-encrypted value. Current FreeScout evidently no longer tolerates that, reads it as empty → 504.
Re-entering the password via the UI is blocked: saving Mail Settings fails with "Domain or IP address is not allowed: 172.18.x.x / fd00:… — Whitelist it via APP_REMOTE_HOST_WHITE_LIST", because the internal relay host mail resolves to private addresses. So affected users cannot even self-repair through the UI.
Reproducible on a fresh installation.

Working workaround:

echo 'APP_REMOTE_HOST_WHITE_LIST=mail,<v4-ip>,<v6-ip>' >> /app/data/env

restart app, then re-enter $CLOUDRON_MAIL_SMTP_PASSWORD in Mail Settings and Save

(CIDR works since FreeScout 1.8.218, so 172.18.0.0/16,fd00::/8 should cover it generically.)

Suggested package fix: ship APP_REMOTE_HOST_WHITE_LIST covering the internal relay in the seeded env, and store the relay password in the encrypted format FreeScout expects. Happy to provide full SMTP logs.