It turns out all I needed to do was restart rsyslog as per https://unix.stackexchange.com/questions/165201/postfix-no-longer-logs-to-var-log-mail-after-it-has-been-deleted-and-re-created (although this relates to Postfix which is not my query)
If your lucky, it's an update of default configuration.
To confirm it, list directories in your /var/mail
You should find a directory for a domain, then a directory for your user like:
firstname.lastname@example.org > /var/mail/domain.tld/user/
A mail directory should have at least directories : cur, new, tmp
Each file in thoses directories are a single mail.
Check into ...
Sendmail by default auto-configures list of local email domains.
It can be tuned off by the following line in sendmail.mc:
You can check list of local email domains using
echo '$=w' | sendmail -bt
You can manually add domains to local email domains via /etc/mail/local-host-names file (one domain per line).
IF you use sendmail (by sendmail.org) command to send email THEN messages are passed to local sendmail daemon listening at 127.0.0.1:25.
It was introduced ib sendmail-8.12 to avoid installation as set root id program. It had been huge security risk.
Your smart host configuration in sendmail.cf should affect next hop.