Generally, if the Mistress sues you for sending a letter (libel) to another person (published) that said she is engaging in sexual activities with your husband (fits the per se categories) and that your claims are false, then you will have to show, by a preponderance of the evidence that your claims are true.
Someone can sue you and mess with your life and emotional status for years while the suit is pending, until you either win or convince a judge to throw out the lawsuit.
She might sue you for an order of protection to keep you away from her, she might see an injunction to keep you from spreading gossip about her. And if you’re not ABSOLUTELY sure, with dates and times and detais of sexual conduct, you might find that when she sues and says through her own testimony as evidence, that your claim is a lie, then perhaps she WINS.
Slander and libel are forms of defamation. Slander is spoken (think “s” is the start of the word), libel is written (we remind ourselves in terms of “l” at the beginning of the word “libel” is for “letters”)… For the most part, there are very few differences in the actual proof of the cause of action for each form of defamation. Publication is required… to a third person… and the sending of a letter to the third party neighbor is certainly considered “publication” (“publication” for the court’s purposes, is simply COMMUNICATING it to someone, it does not require a copyright or book tour to be a “published” libelest!)
Some sorts of lies are considered “libel per se”, meaning that the person suing doesn’t have to present proof of damages. The list of things that are included in “libel per se” tend to include telling people that someone has a vile disease, telling them that someone is promiscuous, telling them that someone has committed a serious crime or is incompetent in their profession.
Every Situation has a slightly different list of “libel per se” options, but most would say it’s “libel per se” when the subject matter of the libelous statement is adultery. This means that the girlfriend would not have to prove any specific damages occurred to her because of your sending letters to other people. To pursue the suit, she ONLY needs to say that she is the victim of you spreading malicious gossip about her and that this gossip is not true.
Truth is an affirmative defense for Defamation. The term “affirmative defense” means that this is a thing YOU, as the defendant, need to take steps to prove. Since you have not given us information on WHY you believe your husband is having sexual contact with this woman, we cannot determine whether your claim that this has happened is reasonable or not.
I need to GENTLY say at this point that even if you are absolutely sure you are right, your own belief system on this issue, as the victim of the illicit affair, are not necessarily enough or are they credible.
YOU are not in a good position to determine whether your own victimhood is objectively reasonable (NO one is in a good position to determin the objective reasonableness of their own victimhood, this is not just about you!)
Ultimately, a lawsuit would take a very long time with you going over & over the issues and making yourself and everyone around miserable, making it very hard to figure out how to overcome this.