Scenario:  If user faces issue in relay of email from SMTP Prevent where sender belongs to a particular domain like xyz.com. User has 2 SMTP Prevent servers configured for load balancing and relay fails only from a particular SMTP Prevent server. However, other prevent servers can reply to the email successfully.  This article will detail how to address such an issue.

1.       It is likely possible that next hop MTAs (downstream MTA) are configured in opposite order to manage load balancing in setup where user has configured 2 mail Prevent servers for load balancing.

2.       In cases where a particular SMTP Prevent fails to relay mail where sender belongs to a particular domain with error message like 554 5.1.0 Sender denied, this smtp banner comes from a downstream MTA and not from mail Prevent.

3.       You can validate this by sending email from a downstream MTA using telnet.

4.       Connect to downstream MTA on port used to relay SMTP traffic (e.g. 25)

[root@hostname ~]# telnet 10.10.0.1 25

5.       Type, helo or ehlo

6.       Type, mail from: <enter mail id having domain which has issue> e.g. [email protected]       If domain example.com is blocked on downstream MTA, it will give an error like 554 5.1.0 Sender denied.

7.       Request customer to get this domain released with help of their MTA administrator.