How to detect and block spam when sending mail?

Question

Email

There is a problem with sending spam via phpmail function. Letters are sent through a smart host, as a result of which it is constantly in BL. How can I check outgoing emails before sending them to the server?

Answer 1

I may not answer the question directly, but still - I think installing captcha on your site will significantly reduce the flow of spam.
Check for at least spamassassin, but this may not be the most recent solution.

Answer 2

look towards spf/dkim