Issue Summary

Statuspage customers are reporting that some of their customers do not receive emails from their custom email addresses, but do receive Subscription Confirmation emails from Statuspage.  

Upon checking, the Customer's clients' email address is being quarantined after a hard fail, because the mail server for the Customer's client is rejecting their email. The error code being reported are variants of "553 5.1.8":

553 5.1.8 Rejected message by MTA, MAIL-FROM domain does not exist in DNS

or

553 #5.1.8 domain of sender address <user@domain.com> does not exist 

This occurs when the customer's custom mail domain does not have an A, AAAA or MX domain entry specified. This is not an issue for Statuspage confirmation emails as we have a MX record associated with the custom domain we send confirmations from. It also does not affect customers whose custom mail name is from their Statuspage domain as that has an A record (after CNAME redirections).

This issue has occurred because of the recommendations for mail server hardening now include checking the validity of the sending domain for emails (through the A,AAAA or MX record existence) and rejecting emails from domains that do not comply. As more companies enable these rules, this scenario is likely to encountered and especially by our Enteprise plan level customers. It is unrealistic for us to expect Customers to have their clients disable this mail server rule.

Currently, Statuspage does not do the following things:

1. Advise in our support documentation that Customers should create a MX record for their custom email-sending domain if they choose one different to their statuspage custom name.
2. Perform DNS checks when a Customer supplies a Custom email domain warning that hardened email servers may reject their emails.

Steps to Reproduce

Create a Statuspage and specify a custom email domain name that has no A, AAAA or MX records associated with it
Deploy a mail server that has been hardened to perform the checks for 5.1.8
Subscribe to the statuspage using an email that will be handled by this mail server
Create an Incident that sends a notification to this particular user

Expected Results

The subscriber will receive emails for subscription confirmation and any incidents and updates created on the Statuspage. 

Actual Results

The subscriber will receive emails for subscription confirmation but not any incidents or updates. Additionally Statuspage will quarantine this subscriber

Workaround

Customers should engage with their own mail server administrators and create a MX record for the custom domain they have specified for their emails from Statuspage. So if they've chosen notifications for status.testcompany.com to come from updates@notification.testcompany.com then they'll need a MX record for notification.testcompany.com