Everytime you receive any email, verify the true sender.
Regardless of detecting email as a suspicious by the spamfilter.
How difficult it is and how to do it exactly depend on the email client. But shortly saying:
See the complete address in the form of somebody@somewhere.domain We do not allow outside sender to spoof @cerge-ei.cz domain.
In case of any doubt, doublecheck it by call or email ask.
Read the next articles as an example how to check real sender. The last article helps you to analyze the complete message path protocol, if you need it.
Do not worry, it seems difficult, but in fact the quick proof can be simple. Is the header too complex, too long and with server addresses from outside of cerge-ei domain? Then it could not be the true cerge-ei message.
How to see the real email sender in Gmail
How to find the true sender in the message header