thanks both.
of course i know that there must be some mail system enabled.
on this linux host is the postfix working, which have as mailname in /etc/mailname set lets say “domain” (there was hostname@domain - which led to format central@hostname@domain - which mail server didnt likes - but as i removed hostname@ and there remain only a domain in that mailname - it at least now feed the teams channel)
all emails by default are send with “whatever@domain”.
so far all is good.
issue is, that site is called “Central” and in past on appliance the nullmailer was set as on picture ^^ nagios-central@domain.
After migration to linux based system from appliance - it send only as “Central@domain” now after update of mailname was made. And I am looking for the solution / config location where i can rename it back to nagios-central (without renaming site) as the “Central” is correct for sitename, but not for “from” field for this nullmailer.
Hope I was able to explain it a little better.