What Service account is used on a stand alone box for SSRS email

If I have a seperate instance of SQL Server installed on a seperate pc and I'm using that as my report server, if I use the build in Local System account in the config manager for the report server, what account is it actually using to send out subscription emails? I ask because I am trying to send emails to groups but I keep recieving the following error.

Your message did not reach some or all of the intended recipients.

  Subject:  Test Report was executed at 5/2/2011 1:22:09 PM
  Sent: 5/2/2011 1:22 PM

The following recipient(s) cannot be reached:

  Test Group on 5/2/2011 1:22 PM
        You do not have permission to send to this recip开发者_高级运维ient.  For assistance, contact your system administrator.
        <exchange server #5.7.1>

I just editted my answer to your earlier version of this question.

SSRS doesn't authenticate when sending email. It uses SMTP. In the "Reporting Services Configuration Manager" you specify the Sender Address and SMTP Server. Test the SMTP connection to this server using this sender address. Sounds like your email server has some relay restrictions.