My VPS server can't send a mail... How to recover

My VPS server can't send a mail... How to recover

Status

1. "The last will keeping place" was not send me a mail.

2. I can't see reCAPTCHA's image at the login screen of "The last will keeping place".

Why? Checking my VPS server and ...

1. change reCAPTCHA public/private-key.

- no good. no effect.

2. change an access-method to reCAPTCHA, from "https" to "http".

- I can see reCAPTCHA's image, but...

- reCAPTCHA verify server doesn't reply. "Can't open socket"

- replace "recaptchalib.php" to the newest version.

- reset from "http" to "https" is going well, but..

- still, reCAPTCHA verify server doesn't reply.

- assign "reCAPTCHA verify server" as IP-address directly, not as an host name.

It's going well.

- And, I think the the DNS service has something wrong,

At this point, reCAPTCHA is well.

3. I check to send a mail from root-user to my gmail account.

- My VPS server can't send a mail. Why ???

- check nslookup, but there isn't "nslookup" in the server! so, I use "ping".

- "mail" command end successfully, but nothing is happen.

4. See many man pages... And check /var/log, /etc files, plesk-control-panel, ...

- restart sendmail, "/etc/init.d/"

That's it. Queuing old mails are flushed!

Result.

Why? - I don't know?

But, DNS and sendmail has something wrong.

I needed restart "sendmail".