I am currently facing an issue with the simple Sendmail on CentOS 7, and I am seeking your assistance to resolve it.
The problem is that although we have enabled Sendmail in the (etc/php.ini) file and configured it, emails seem to be stuck in the mail queue (mailq) and are not being delivered. I have verified that Sendmail is enabled and functioning and Postfix has been started and configured.
However, despite these efforts, the emails are not reaching the recipients. I have checked for errors, but unfortunately, I am unable to identify the root cause of the problem.
If you have experience with Sendmail on CentOS 7, or if you can provide any guidance or troubleshooting steps to resolve this issue, I would greatly appreciate your assistance. 
First of all, the information you provided has nothing to do with PHP, but the following are obvious
You have tried to send an email to [email protected], which the system faithfully tells you that user with the name "subject" does not exist (did you create a user with the name "subject" on your localhost ??)
You have tried to send an email to [email protected], which the system faithfully tells you that user with the name "test" does not exist (did you create a user with the name "test" on your localhost ??)
You have attempted to send an email to a mailbox of an external domain: [email protected], but the recipient mail server checked that the sender is [email protected], so it rejected it because of 550 verification error (most likely the recipient mail server would check and reject email which originates from a sender without a valid MX record in the domain name, which is of course true because your domain is just localhost.localdomain)
By the way, please note that sendmail and postfix are TWO DIFFERENT daemons.