Option "Do not modify /etc/mail/local-host-names file automatically" is enabled for Container in PBA-S:
Top > Service Director > Virtuozzo Manager > Containers > #CTID > General Settings --->8--- Do not modify /etc/mail/local-host-names file automatically [OK] ---8<---
What does it mean?
It's by design that file /etc/mail/local-host-names is used by Sendmail for storing list of local domains for accepting emails for.
PBA-S changes the file when client manages mail service setting through CP (Control Panel Home > System > Mail Management > Mail Server): adds new domains, domain aliases, etc. The option prevents file modifications by PBA-S.