All of these are god suggestions - but in my experience, especially since
I my self don`t know anything
about managing dedicated server.someone manage my server voluntarily
You're on unsure ground, there are three avenues I would look for first.
1. As mentioned: Do you have **any** autoresponders?
From:my-intended-attack@example.com
To:your-easy-to-guess-account@yourexample.com
Someone sends a thousand of those and they bounce the autoresponder to my target, using you as the mail server. Kill all autoresponders.
2. Do you have any free mailer scripts on your site?
Many of these are extremely sloppy, and allow me to do things like this:
to: 1@example.com,2@example.com,3@example.com,4@example.com,5@example.com,6@example.com . . . .
Or some other vulnerability, like modifying headers to add a bcc field, submitting multipart input, there are more. Look to your mailer scripts.
3. How well do you trust this volunteer, and how skilled are they? They should be able to determine the source in an hour or less, and put a stop to it in most cases. I hate to sound paranoid, maybe they are taking advantage of the situation. See what they can do to help you.