If contact forms have been the preferred option, as a means of contacting the managers of a website, it has been because publishing an email address on the Internet is opening the door to receiving a large amount of spam or unwanted emails. It is a fact that crawlers operate on the network that go through websites, collecting email addresses, which they include in databases, that are sold to send advertising, or any other type of messages with unlawful purposes. Or, they use the contact forms in an automated way, to send those unwanted messages.
Implement a reCAPTCHA, to verify that it is a person. Still, it is not enough, because there are trackers capable of overcoming this control.
Use a service to verify that the sender's email address and IP are not registered in a database, which collects information from those spammers. Our recommendation would be Stop Forum Spam, but there are others like CleanTalk or Akismet, widely used with Wordpress.
Analyze the text of the message and if some words are detected, often used by spammers and without any relation, with the vacation rental business, the sending of the message can be stopped.
When from the server, it is detected that a message may be fraudulent, in addition to preventing its sending, we will redirect the user to the contact form, where we will inform them of the reason why the message has not been sent. In this way, if the message was not fraudulent, the client would at least know why it could not be sent.