Undelivered Mail Returned to Sender

commerciale@etruriadesign.it, ERROR CODE :550 - "The mail server detected your message as spam and has prevented delivery."

I have been corresponding with the receiver and they wrote

"Ciao, ho fatto verificare ma purtroppo non è un problema che deriva dal nostro server ma dal tuo, quindi dovresti segnalarlo a chi ti segue l’email e vedere se riescono a sbloccarlo"

"Hi, I had it checked but unfortunately it's not a problem that comes from our server but from yours, so you should report it to whoever follows your email and see if they can unblock it"

what do i need to do to resolve it?