Occaisonally, we will receive an email in support saying "Undelivered Mail Returned to Sender". Sometimes it is due to an incorrect email, but sometimes there is an issue with an improper DNS configuration. If the customer calls and lets us know they are having an issue not receiving emails or with bounced back emails, you can let them know the following:
550 invalid DNS MX or A/AAAA resource record
This error indicates that there is a misconfiguration with the MX (Mail eXchange) record. The MX record is a DNS record which directs incoming mail to the proper mail server. The improper configuration could be the fault of the sender of the e-mail, or the intended recipient of the e-mail. Both should check to ensure that the MX record is NOT configured to use a CNAME (also known as an alias), otherwise the mail will not be deliverable.
This improper DNS configuration is not something that can be altered in a mail program or on your current computer or device used to send/receive e-mail. Contact your webmaster, web developer, or website administrator and ask that the recipient contact their's as well.
The customer will need to take action to have this fixed on their end.