You may see errors like the following in the Exim log showing that mail for a domain is being proxied to another server, even after running the unset_all_service_proxy_backends API call.
2020-10-11 06:05:43 1kRTXa-008CrU-W2 ** firstname.lastname@example.org R=manualmx T=remote_smtp H=newhost.domain.tld [198.51.100.2] X=TLS1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=yes: SMTP error from remote mail server after RCPT TO:<email@example.com>: 550-Please turn on SMTP Authentication in your mail client.\n550-oldhost.domain.tld [198.51.100.1]:12345 is not permitted to relay\n550 through this server without authentication.
2020-10-14 09:43:45 1kSdJw-0066xQ-Bb == firstname.lastname@example.org R=manualmx T=remote_smtp defer (0) H=newhost.domain.tld [198.51.100.2]: SMTP error from remote mail server after initial connection: 451 Temporary local problem - please try later
This is caused by the unset_all_service_proxy_backends API call not removing the domains from /etc/manualmx file. Exim is not included in Service Proxying as documented here.
The entries can be removed from /etc/manualmx to stop proxying email. The article below describes this further.