Several services are configured by puppet with a mail server of mail_smarthost. This selects only the first mail server from a per-site ordered list of mail servers (defined in manifests/realm.pp), and results in an SPOF.
To address this I propose configuring these services to use the exim localhost SMTP listener. This moves the SPOF to the local MTA which has no external networking or service dependencies beyond the locally running daemon. The local MTA then handles relay to the appropriate smarthost with more robust queueing and failover.
This is a follow-up from T196598: Phab and Gerrit emails stopped at around 1900 UTC 6th June.