Well, move on mentally (you are a better person etc.).
But one might argue that it is your duty not to let it go, for the sake of the system (i.e. others). How about:
1. Send the client a non-aggressive email reminding them of the facts and asking for the payment + interest. Do not make any threats.
2. Find out (e.g. contact a local lawyer) about the admissibility in court of emails that you have. You need to prove that the client did actually send them. Find any other circumstantial evidence (code, phone calls) you have that argues for the authenticity of those emails.
3. If it works that way in your jurisdiction, argue that they acted in bad faith to claim damages.
4. Likely not doable, but if you can argue that they did not intend to pay from the beginning, get them accused of fraud.
All that would work where I live overseas, I do not know if it will work where you live.
But one might argue that it is your duty not to let it go, for the sake of the system (i.e. others). How about:
1. Send the client a non-aggressive email reminding them of the facts and asking for the payment + interest. Do not make any threats.
2. Find out (e.g. contact a local lawyer) about the admissibility in court of emails that you have. You need to prove that the client did actually send them. Find any other circumstantial evidence (code, phone calls) you have that argues for the authenticity of those emails.
3. If it works that way in your jurisdiction, argue that they acted in bad faith to claim damages.
4. Likely not doable, but if you can argue that they did not intend to pay from the beginning, get them accused of fraud.
All that would work where I live overseas, I do not know if it will work where you live.