Form not working – not receiving the emails

I was just told some clients used the forms on a website I support but the client never got the emails. I looked into it and found it looks like it is working but no emails are being received. I did make sure my “to” and “from” are from the same domain. The compatibility check works but not emails. Also talked to a friend who is having the same issues with his websites and he maintains a lot of websites. Can someone please help?

https://www.ads-software.com/plugins/contact-form-7/