Hostname immediately under TLD possible?

[hruodr]

Are hostnames immediately under the TLD allowed?

Technically it works, I use a name like xxxx.xx, not host.xxxx.xx

And yes, I put subdomains of that also. And I am sure not the only one,
do for example "drill ibm.com" and you get an A record.

But I read everywhere that there must be at least three labels:
hostname, and domainname with TLD.

Where is this defined?

I suspect a provider of free mail sets the mails from my server on spam because of
this. And this free mail provider is not google.

 

[Alain De Vos]

hostname
-> freebsd.ala

/etc/hosts
->
::1 localhost localhost.my.domain freebsd.ala
127.0.0.1 localhost localhost.my.domain freebsd.ala

/var/unbound/lan-zones.conf
->
local-zone: "ala." static
local-data: "freebsd.ala. IN A 127.0.0.1"
local-data: "freebsd.ala. IN MX 10 ala."
local-data-ptr: "127.0.0.1 freebsd.ala."

 

[SirDice]

I suspect a provider of free mail sets the mails from my server on spam because of
this.

I doubt they're being blocked because they came from someuser@somedomain.tld. That would block 99.9% of all valid email. Is there anything being returned? A reason? Or do they just disappear? They could be blocking based on somedomain.tld not having an MX record and they might consider that not a 'real' email domain. Or your reverse lookups fail for some reason, or return something else entirely.

 

[hruodr]

I doubt they're being blocked because they came from someuser@somedomain.tld.

No, I am not speaking on domain of email addresses, but the hostname
of the MTA.

I have MX records for the mail domain that in this case coincide with
the hostname.

I have A record and PTR (reverse DNS) for the hostname.

The problematic freemail is:

Email policy | WEB.DE Postmaster

On this page you will find the guidelines that you as a sender must take into account in order to be able to successfully send e-mails to WEB.DE.

postmaster.web.de

I quote:

(rDNS) For your mail server's IP address, there must be a valid reverse DNS entry in the DNS that points to your domain. The rDNS entry must be set to an individual, complete host name (FQDN).

They put in their footnote 3:

A Reverse DNS entry or FQDN (Fully Qualified Domain Name or PTR-RR) is the unique name of an internet host. The FQDN can be used to discover the host's IP address. The Reverse DNS entry should be used as the HELO when sending emails. You can find detailed information in the Digital Guide from IONOS.

And the linked "digital guide":

The fully qualified domain name consists of three or more labels: The top level domain, the domain names, optional subdomains, and the host name.

With sendmail I could use in sendmail.mc

define(`confHELO_NAME',domain)

but I am not enthusiastic to change my reverse DNS only due to strange
behaviour of mail providers.