Question SPAM via my server

[Pecnet]

good evening everyone,
I would like to have your help with a problem I encountered today.

My server, my host and an email that I own sending emails without my consent

I don't know if it's a php script or if a hacker managed to use smtp parameters etc ... but I don't know what to do to avoid this.

By searching on google I found this ->

Protéger contre le spam sortant

Si votre hébergement inclut des services de messagerie,n’oubliez pas que votre serveur de messagerie peut être utilis...

docs.plesk.com

I added the sending limit, it looks like it works

but I don't know if the problem comes from a hacked email account or a php script

Can you give me some leads to follow to know if the problem comes from a smtp account or from a sending form in php

Maybe give me other advice?

Thank you