Regarding your example and please don't take this advice as absolute but Nokia is a made up word and is not a dictionary word so even if you would sell or advertise other goods as mobile cellphones you would still have to hand over the domain.
You are in this example infringing on their trademark.
I think what you are refering to is for example the Apple case where Apple computers and Apple music where having differences on trademark issues.
These two companies are both called Apple but Apple is a generic word and as long as the two companies have both different business practices both can continue to operate under the name Apple.
The trademark issue that was recently going on between these companies is because Apple computers was getting into the music business and thereby causing confusion since Apple music was already involved with music.
As long as two companies with similar generic business names have different business pratices they are allowed to operate under this generic name.
But if one is entering the business field of the other similar generic business name holder this business is infringing on their trademark.
Nokia is a made up word and can be claimed by a single business and can also be enforced as such that it is only intended for one business only.
For instance Google is a made up word and no other company/individual may offer business of any kind or services under the name Google or have the name Google incorporated in their business name.
In your case you could use dfe.de for other type of business that the current trademark is involved in.
As long as you are not involving dfe.de with these practices:
Nice Classification: 16
List of goods and services Printed matter, books, brochures and periodicals relating to succession rights and inheritance tax rights.
Nice Classification: 41
List of goods and services Arranging and conducting of congresses, meetings, seminars and comparable further training events relating to internation succession rights and inheritance tax rights.
you should have nothing to worry about.
Claiming rights to an acronym is even a harder challenge for the claiming party and i don't think they will have a chance of taking this domain from you.
You could for instance have the domain UPS.com and start a site that is about Underwear-Pants-Shorts and you would have legitimate use for this domain and UPS would not have a chance of taking this domain from you.
At least this is my perception if i am wrong please inform me about this fellow members.
But like Labrocca said you better be safe then sorry and take precautions as soon as possible.
Then i would try do more research on the matter and whatever you do don't offer the domain for sale for a while unless you truly know the other party is not a business that can claim trademark rights to it.
I think they have no case against you since you are not in the same type or planning to be in the same type of business they are involved in.
Good luck!
