[CentOS] Mail has quit working

>
> Did the telnet localhost 25, it answered but not sure I understood you
> what I needed to do.
>

​type all this in once connected, substituting as necessary & ignoring the
comments in ()

helo mydomain.com (substitute a valid domain here)
mail from: <someuser at validdomain.com>
rcpt to: <youremailaddress at yourdomain.com>
data
From: someuser at validdomain.com
to: youremailaddress at yourdomain.com
subject: test
  (note blank line here)
This is a test message
.