Hello I figured out what was wrong - of course it had to be the encoding, but I tried iso-8859-16 and there were problems with a few characters; I thought that somehow Usermin is limited to ISO encodings and not Unicode aware because I originally wrote "UTF-8", with caps, and it didn't change a thing.
Today by pure accident I tried in Preferences>Sending e-mail>Character set for sent mail>utf-8 (small letters!) and it works! Thanks for the support anyway.
And now the obvious problem - I have lots of users - how do I change it for everyone, to become a default, sort of speak? It's not an option to make it individually, nor to explain to everyone, what's an char encoding, where to go and stuff - people become scared when they hear tech talk and "settings".
The second question: how do I translate the Menu and stuff in Usermin, where is the romanian language file located? Now it's incomplete.