We have identified an issue with exim rejecting what are valid IPv6 representations in the generated /etc/exim.smtpauth file. This problem manifests with errors in /var/log/exim_paniclog (on cPanel) as:
2019-07-18 08:00:22 Internal error: invalid IPv6 address "2001:470:1f0b:a9:9dc3:6ed8:e840::/106" passed to host_aton()
This innocuous error actual causes exim to defer (error 421) all email originating from IPv6 sources. This prevents its delivery inbound.
While we look into mitigating the issue exim has with the generated IPv6 CIDRs we would suggest disabling SMTPAUTH_RESTRICT/CC_ALLOW_SMTPAUTH and removing the exim.conf line used to refer to it (see csf docs).
This issue has now been fixed in csf v13.04.