I was called by R (guy I work for) to double check the email system since it looked like (to him) that it wasn't working at all and no emails were being sent (order confirmations, etc). I start checking, and from what I could tell, the email system was working fine. I placed a few test orders from a few sites and got the confirmation email. And from the mail logs, it looked like it was sent off to R as well.
But he wasn't getting them.
A few more tests while talking to R seemed to confirm that email sent to him from the server was being accepted by his ISP (Internet Service Provider), but R was still not getting them. Which indicates to me that his ISP is filtering out spam and for whatever reason, email from this server was being flagged as spam and not delivered.
I then did the following on the server to check something out:
>
```
[root]rnm.miami:~>hostx -t mx XXXXXXXXXXXX
XXXXXXXXXXXX MX 20 mail2.XXXXXXXXXXXX
!!! XXXXXXXXXXXX MX host mail2.XXXXXXXXXXXX is not canonical
XXXXXXXXXXXX MX 10 mail.XXXXXXXXXXXX
!!! XXXXXXXXXXXX MX host mail.XXXXXXXXXXXX is not canonical
```
Hmmm … odd. Double check the zone file.
>
```
$TTL 3600
@ IN SOA ns1.XXXXXXXXXXXX. root.XXXXXXXXXXXX. (
2004091801 ; Serial
10800 ; Refresh
3600 ; Retry
3600000 ; Expire
3600 ) ; Minimum
IN NS ns1
IN NS ns2
IN NS ns3
IN MX 10 mail
IN MX 20 mail2
mail IN A XXX.XXX.XXX.XXX
mail2 IN A XXX.XXX.XXX.XXX
ns1 IN A XXX.XXX.XXX.XXX
ns2 IN A XXX.XXX.XXX.XXX
ns3 IN A XXX.XXX.XXX.XXX
```
Nothing out of the ordinary. The RR (Resource Record)s for mail and mail2 aren't CNAMEs (which is a common mistake). They look fine to me, and running host on outside machines report back fine:
>
```
[root]linus:~>host -t mx XXXXXXXXXXXX
XXXXXXXXXXXX MX 20 mail2.XXXXXXXXXXXX
XXXXXXXXXXXX MX 10 mail.XXXXXXXXXXXX
```
A quick Google [1] didn't reveal anything I already didn't know (most of the problems I found were CNAME errors). So, I started playing around with the zone file. Make a change, same error. Make another change, same error. It was only when I removed all references to mail and mail2 when I discovered the problem.
By then, I was doing hostx -G -S -C -A -L 1 zone (which verifies the entire zone) and getting back errors about mail and mail2. Which shouldn't be happening, as I had just removed all references to them in the zone file. There was only one other place where they were defined—/etc/hosts. Once I removed those entries from /etc/hosts, the problem went away:
>
```
[root]rnm.miami:~>hostx -t mx XXXXXXXXXXXX
XXXXXXXXXXXX MX 20 mail2.XXXXXXXXXXXX
XXXXXXXXXXXX MX 10 mail.XXXXXXXXXXXX
```
Talk about your overly pendantic host command.