3 Questions.
1) Is the word they claim you're infringing a generic word?
2) Are you and the complainant residing in the same country?
3) If matters do escalate, are you willing to hire a lawyer to fight for your cause?
You may want to read this:
https://forums.digitalpoint.com/threads/udrp-claim-domain-registered-before-trademark-filed.1540208/
There are 2 possibilities that will happen now:
1) they can sue you for infringing on their trademark, but not for cybersquatting as you didn't fulfil the 'criteria' of being one. You will be asked to attend court. If you're living from a different country than the complainant, then you can ignore them. But if you're living in the same country with the complainant, then you will need a lawyer, one who specialises trademark laws, to defend you in court (even if you've said that you registered the domain name before the trademark was filed).
2) They will file a UDRP with WIPO. The complainant will file a UDRP with WIPO (I think it costs $500-1500 depending on where they file it. Any additional judges/panellist will incur a price and it will be split between you, the defendant, and the complainant.) and you will be asked to respond within a certain amount of days as the 'case' will be reviewed by a judge to decide the outcome. You can submit a defendant reply yourself but many would advise you from doing so. And if you do get a lawyer, you must find one who specialises in cyber law. You could even request for multiple panel judges to decide the outcome (1 to 2, 3 or even 5... I remember it can be numerous). The panellists will read your case and vote on the outcome.
That is why you should ask yourself is it worth it to fight for it, especially since you've not put up any content and not monetising at the moment.
What I worry most is if they wants to bring you to court for trademark infringement... and say you lost the case (touch wood) you will be fined heavily and have to surrender the domain name. However, like you've pointed out, you registered the domain name BEFORE the trademark was filed. Again, you will need a lawyer to defend you in court and that means $$$. If they only file a UDRP, the most you will lose is the domain name and pay your registrar a small fee, and I believe you're pretty much safe then.
EDIT: Oh wait, seems like there's a change of law:
https://trademarkwell.com/no-you-cant-get-a-registered-domain-name-by-registering-a-trademark-now/
If you're daring enough, you can reply back to them and quote all these laws that others and me have posted. Tell them that they've no grounds to sue you since you've registered the domain name before the trademark was filed. Then, if they concede defeat, you could tell them they can buy your domain name at a 'reasonable' price if they want.
