Mail errors · Microsoft 365 and Outlook

What does “5.7.25 Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record” mean?

The server sent over IPv6 from an address with no PTR record.

Last read from the sources at the foot of this page.

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Microsoft bounce
Microsoft Outlook
5.7.25

Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record

As Microsoft prints it.
01

What does it mean?

Reverse DNS turns the address back into a host name. Microsoft requires it for mail over IPv6.

02

What exactly does the source say?

5.7.25 Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record The sending IPv6 address must have a reverse DNS record to send email over IPv6.

03

Why would a small firm see it?

A server that sends directly over IPv6, often one set up by a web host.

04

How do I fix it?

  1. Find the server from the bounce.
  2. Send through your mail provider instead, or have the company that holds the address set a PTR record.
Sources

Where does this come from?

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