I just had a really alarming situation with one of the subdomains.
So the problem was that one of subdomains mail.xxx.pl was unavailable (server request failure, no ping, no dig response), when all other subdomains were fine (the same domain of course).
All subdomains pointing the same server, and are as A entry. Domain itself were good too, name servers are OK, and were responsing good.
So somehow just one subdomain couldn't have been resolved. (it was working all few weeks already, and I haven't change any settings, it just stopped working with no reason).
Is this possible that somehow on the higher level, got wrong? Because I really can't imagine what happened.
I was trying to look logs, and few minutes later it just started to work again.
I would appreciate any clues where to check or what could have happen.