I had a look.
The resolvers chose 2a03:b0c0:2:d0::e83:6001 as resolved address (one of four) which currently is not responding. Neither on my local machine nor in our environment.
We have no control over incorrect DNS entries or machines that are down.
There is no point testing the email now as I have moved the email provider this morning.
However at the time of the error it was working and fully functional. Without problems added another email hosted at ZOHO.
I do not understand for what reason one email server may be working fine while other not, it does not make good impression of Zammad so I am considering now Resource Tracker as an alternative.
Thank you anyway.