Issue:
When sending an email to a recipient hosted by 1&1 you receive a bounce message including the following:
550-Requested action not taken: mailbox unavailable 550 invalid DNS
Solution:
This error is normally generated when your domain includes one or more MX records that are CNAME records. Internet email standards do not permit the use of CNAMES in MX records. To resolve the issue you will need to remove the CNAME from your MX records and replace with a valid A record.