[exim] 454 Error

Hi folks,

Uber users can initiate support tickets by replying to certain emails; the
replies go to some_unique_address@???. We've recently received a
report from a user of EXIM that their emails were bouncing when sending to
this domain.

help.uber.com is a CNAME to frontends.uber.com which is a CNAME to
frontends-sjc1.uber.com which has an MX record of mx.sendgrid.net.
Sendgrid is our inbound SMTP provider:

```
kadin-C02L70PZFLCG:~ mikekadin $ dig help.uber.com MX

; <<>> DiG 9.8.3-P1 <<>> help.uber.com MX
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 13008
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;help.uber.com. IN MX

;; ANSWER SECTION:
help.uber.com. 60 IN CNAME frontends.uber.com.
frontends.uber.com. 60 IN CNAME frontends-sjc1.uber.com.
frontends-sjc1.uber.com. 300 IN MX 0 mx.sendgrid.net.
```


The user who reached out said that they are using Exim 4.87 and had the
following results when sending a test email to test123@???:

```
Recipient: test123@???
Router: dkim_lookuphost
Transport: dkim_remote_smtp
Out Time: Jul 21, 2016 1:13:13 PM
ID: 1bQHXO-00020O-59
Delivery Host: mx.sendgrid.net
Delivery IP: 167.89.125.4
Size: 39.83 KB
Result: 25: SMTP error from remote mail server after RCPT TO:<
test123@???>: 454 4.7.1 <test123@???>: Relay
access denied
```

Do you think Uber's DNS configuration for help.uber.com is problematic? Is
this the expected behavior for Exim? Or is it possible that this Exim
instance has some kind of configuration that could cause this?


Thanks for your time,

--
Mike Kadin
kadin@???