I did forget to create the user. Now, I don't get the error message anymore. Thanks.

Help,

 I have added a new domain in my Core Module configuration of Mercury. In the local domains tab, I entered:

Local host or server: DM=elink

Internet name: elink.xxxxx.xx.xxx.com (masked real values)

According to the help, 

With this entry in place, mail sent to [any address]@fish.net will be delivered into user mailserver's mailbox.

Whenever I try to send mail to an address at that server, I get the message:

 User MzYwNzMA@elink.xxxxx.xx.xxx.com not known at this site.

Why am I getting this message?

>  I have added a new domain in my Core Module configuration of Mercury. In the local domains tab, I entered:
> Local host or server: DM=elink

Does the user ELINK exist?  

If you just created the user did you use the CTRL+ Configuration + Manage local users?  

> Internet name: elink.xxxxx.xx.xxx.com (masked real values)
> According to the help,
> With this entry in place, mail sent to [any address]@fish.net will be delivered into user mailserver's mailbox.
>
> Whenever I try to send mail to an address at that server, I get the message:
>  User MzYwNzMA@elink.xxxxx.xx.xxx.com not known at this site.
> Why am I getting this message?

Not sure, show us a copy of the [Domains]  section of the Mercury.ini.  The real values please, putting a domain name into a message is not going to change the amount of spam you get one iota.