Alert Mail when virtual server down trubleshooting

I use this script on /config/user_alert.conf 

alert VIRTUAL_SERVER_DOWN "/Common/gslb_test_vs has become unavailable" {
 snmptrap OID=".1.3.8.1.4.1.3475.2.4.0.500"
}

alert BIGIP_TMM_TMMERR_LAST_PMBR_DOWN {
    snmptrap OID=".1.3.8.1.4.1.3475.2.4.0.500";
    email toaddress="netinfra@example.com"
    fromaddress="root"
    body="Virtual Server Down"
    }

But when I capture the packet here to_address(postmaster@hostname.example.com) is showing. I am expecting here: netinfra@example.com

10.10.10.208    10.10.10.225    SMTP    145    C: HELO hostname.example.com

10.10.10.225    10.10.10.208    SMTP    157    S: 250 mailserver.example.com Hello [192.168.1.208]

10.10.10.208    10.10.10.225    SMTP    157    C: MAIL FROM:<root@hostname.example.com>

10.10.10.225    10.10.10.208    SMTP    124    S: 250 2.1.0 Sender OK

10.10.10.208    10.10.10.225    SMTP    161    C: RCPT TO:<postmaster@hostname.example.com>

10.10.10.225    10.10.10.208    SMTP    170    S: 550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain

netinfra@example.com is our group mail. Is there anyone face this issue and how can I solve this issue.

Thanks in advance :)