I wanted to share a ‘gotcha’ I encountered recently when a customer I had moved to Office 365 was getting an Non-Delivery Reports (NDRs) when trying to send emails to some recipients:
Sender address rejected: Malformed DNS server reply
I’d never since this before and was curious as to what the problem could be, given that everything was working fine for the customer.
It turns out that somehow copy and pasting the MX record from the Office 365 portal to the domain name hosts DNS editor had carried with an invisible character at the end (a tab) which I deleted and all was well.
I was very surprised that the dns editor didn’t ‘TRIM’ the left and right side of the MX record entry (let alone just flag any invalid characters).