Not sure but I would guess that MX records for the recipient domain still points to a mailserver (presumably at the hosting company) where that domain now has been removed.

One of my users started receiving the following error when attempting to send to an address that she did not have problems with last week.

550 5.7.64 TenantAttribution; Relay Access Denied

Can anyone explain what is causing this?  MercuryE is handling the sending, Pegasus Mail is the client.

Update:  I have since found out that the phone has been disconnected at the business these emails were being sent to.  Users says email sent last week were not responded and these were follow-ups.  I now assume this business has closed.  I'm still curious about this delivery failure message though.