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The simpliest answer: because it's done so

You can create ptr to put to any dns name.

Do not need to follow the pattern:

A record --A-resolving--> ip address
ip address --PTR-resolving--> A record

You can do so, but do not have to. You just need to have a PTR record.

The full answer:

Some mail servers (public especially) have more than one domain. But you cannot create several PTR record to single ipip*.

* see comments below

The simpliest answer: because it's done so

You can create ptr to put to any dns name.

Do not need to follow the pattern:

A record --A-resolving--> ip address
ip address --PTR-resolving--> A record

You can do so, but do not have to. You just need to have a PTR record.

The full answer:

Some mail servers (public especially) have more than one domain. But you cannot create several PTR record to single ip

The simpliest answer: because it's done so

You can create ptr to put to any dns name.

Do not need to follow the pattern:

A record --A-resolving--> ip address
ip address --PTR-resolving--> A record

You can do so, but do not have to. You just need to have a PTR record.

The full answer:

Some mail servers (public especially) have more than one domain. But you cannot create several PTR record to single ip*.

* see comments below

1
source | link

The simpliest answer: because it's done so

You can create ptr to put to any dns name.

Do not need to follow the pattern:

A record --A-resolving--> ip address
ip address --PTR-resolving--> A record

You can do so, but do not have to. You just need to have a PTR record.

The full answer:

Some mail servers (public especially) have more than one domain. But you cannot create several PTR record to single ip