Emails not delivered - unavailable invalid DNS MX or A/AAAA resource record

Question

I have set up Azure Communication Service for email sending, with a custom domain.

Emails are delivered successfully to some email addresses but not others.

The error I get is:

Requested action not taken: mailbox unavailable invalid DNS MX or A/AAAA resource record.
The email could not be delivered because the sender domain is not configured in accordance with RFC. We expect an MX record which itself has a valid A record.

I have no requirement for inbound email, and Azure Communication Services does not support inbound email. However the mail server seems to be expecting an MX record which is normally used for inbound email.

What is the solution for this using Azure Communication Services, as currently it is not reliable as an email sending service?

Answer 1

Hi @Matt Atkins , I'm sorry to hear that you're facing this issue.

Your understanding is correct; MX records are used for inbound emails. However, some mail servers check the sender domain's MX record as an anti-spam measure. If the sender does not have an MX record, the email may be rejected.

To resolve this, you need to add a valid MX record to your custom domain. Since ACS does not currently provide an MX record, you will need to obtain one from your domain registrar. Alternatively, you can check with the receiver to see if there is a way to exempt emails sent from your domain from this MX record rule.