A problem with the contact form on my website

A problem with the contact form on my website

I'm not sure if this relates to Zoho mail or not, but I wanted to ask help for this issue here. I finished changing the mail server to Zoho. I changed MX record, SPF, and DKIM. Everything is working now as expected except the contact form on my website. When anyone fills the form and hits submit, it shows an error message. Which means it is not able to use the mail address something@mydomain.com to send messages. Is this because the mail and the domain are hosted on two different servers or what?? I contacted my domain host many times and everytime they say they make a test and it is working with no issue. I contacted them again and again and nothing changed. I reinstalled and reconfigured the plugin and this didn't succeed. What is going on?? I'm stuck. Please, if anyone can tell me how can I fix this, I'll be grateful.