Originally we had the following configuration for our e-mail system: domain1.com and domain2.com where pointing their MX records to our router1. Later we noticed that there were too many e-mails coming to non-existing accounts for domain1.com and here is what we did. We set up a Linux Sendmail server, created aliases for domain1.com that would point to domain2.com (e.g. “aliases” file: user@domain1.com: user@domain2.com). We also put this Linux box behind router2 and re-directed domain1.com’s MX record to router2. Now, theoretically any mail for domain1.com has to go to router2 and be forwarded if a record exists in “aliases” file and dropped if it did not. It works fine for legitimate mails. However, we are still getting e-mails for non-existing users for domain1 and they go directly to router1, as if someone has statically assigned resolution of domain1.com to original router1. I get response from router2 when I ping domain1.com, so it is kind of a mystery that has no explanation.