cname replace in mail address? [off-topic] (Re: Attn Ronald Klop)

[Ronald Klop]

offtopic, but why do some mailers replace a CNAME in a mail-address?

root at sheeva2:/var/vmail# host klop.yi.org
klop.yi.org             CNAME   thuis.klop.ws
thuis.klop.ws           A       212.123.145.58

It is not the first time that I'm bitten by this, but I never understood  
it.

Ronald.

On Tue, 24 Aug 2010 22:05:46 +0200, Andriy Gapon <avg at icyb.net.ua> wrote:

>
> Ronald,
>
> your email address bounces, that's inconvenient.
>
>
> -------- Original Message --------
> Subject: Returned mail: Service unavailable
> Date: Tue, 24 Aug 2010 23:03:33 +0300 (EEST)
> From: Mail Delivery Subsystem <MAILER-DAEMON at citadel.icyb.net.ua>
> To: <avg at icyb.net.ua>
>
> The original message was received at Tue, 24 Aug 2010 23:03:27 +0300  
> (EEST)
> from porto-e.starpoint.kiev.ua [212.40.38.100]
>
>    ----- The following addresses had permanent fatal errors -----
> <ronald-freebsd8 at thuis.klop.ws>
>
>    ----- Transcript of session follows -----
> ... while talking to thuis.klop.ws.:
>>>> RCPT To:<ronald-freebsd8 at thuis.klop.ws>
> <<< 554 5.7.1 <ronald-freebsd8 at thuis.klop.ws>: Relay access denied
> 554 <ronald-freebsd8 at thuis.klop.ws>... Service unavailable