if a send 1000 mails then i get 500 mails back and the spam

You are getting spammed and/or replied by bounced email. The first example is a typical automatic reply by standard mail boxes.

It is a good idea to setup your mail box in a way to allow sending but not receiving email on the board's mailbox, or at most, to accept bounced mail into a specific recipient.

In any case, 1000 email are not considered spam. Most ISP start to look at you when you exceed 100.000 daily.

thx but where can a install that option ?

On my server I have a mailbox which allows relaying (sending email from any mailbox), and the return mailbox, specified in vBulletin as the webmaster's email does not exists. Thus, all returning email is rejected.

As a new option, you may now specify the return path in the vbulletin email settings (Bounce Email Address). Thus it was made possible to allow bouncing email to be received by a different mail box. I'm not sure since when this option was introduced, but I suspect it was new to 3.7.3; it needs the SMTP settings switched on in order to work.

thx for this tip