Need more information to give an intelligent response. In what capacity is the trust suing you? What is your relationship to the trust? What is the trust's relationship to FL? There are very specific limitations to the granting of venue and jurisdictional requirements must be met in order for a foreign trust to file a complaint in Florida (see Section 736.0204 and 0205 for more information). If a complaint has been filed against you, do not delay and risk defaulting - contact a trust attorney immediately.
Information provided here is anecdotal and should not be relied upon or considered legal advice. Every matter is different and answers given here are general in nature and may not reflect current Florida law at the time you are reading this posting. Please contact an attorney for legal advice concerning your matter.
The trust only needs to be the owner of a Florida asset that would have a reason to sue you, or be a Florida trust (which could be an out of state trust that just appointed a Florida trustee so as to avail themselves of Florida law, and become "a Florida trust"). Florida trusts are not necessarily registered. I recommend you take this seriously and address the claim.
This is off-the-cuff advice and does not establish a client-lawyer privilege. Nothing I say here can be used to suggest the avoidance of taxes/penalties/interest due. I am only licensed to practice in the state of Florida.
If the trust (trustee of that trust) is not a party with standing to sue you, that can easily be addressed by a motion in court. You really don't have much choice but to talk with a civil litigation attorney here because I would hate to see you default on a claim that should have never been made against you in the first place.
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This is a general discussion about legal questions and not a legal consultation. No attorney/client relationship exists. For a specific answer and advice on your specific legal matter, you should arrange a consultation with a practicing attorney.