Setting "auth_cache_verify_password_with_worker = yes" in order to leverage multiple cores for the Dovecot authentication-process causes Dovecot not to proxy anymore. With debug-logging I figured:
auth_cache_verify_password_with_worker = no
passdb out: OK 1 user=username host=bla port=10993 ssl=any-cert mail_crypt_global_public_key=key mail_crypt_global_private_key=otherkey hostip=123.123.123.123 proxy pass=pw
auth_cache_verify_password_with_worker = yes
passdb out: OK 1 user=username
The rest seems missing.
Dovecot version: v2.3.11.3
Searching for this issue, I found at least two posts mentioning the same symptoms:
https://dovecot.org/pipermail/dovecot/2018-April/111583.html https://dovecot.org/pipermail/dovecot/2020-April/118564.html https://listen.jpberlin.de/pipermail/dovecot/2020-April/001915.html (same as the english one from april 2020)