How do I receive email via SMTP on TDC’s residential ADSL lines?

TDC’s residential ADSL products all block external connections to port 25, which is used for SMTP.
SMTP must be used if you wish to run your own mail server and thereby host your own domain. This may be desirable if you need to use a large number of email addresses or similar.

This restriction means that other email servers around the world cannot connect to email servers running on TDC’s private ADSL products.

If you still wish to receive email, this can be achieved by adding the following lines to your DNS:

yourdomain.dk. IN MX 10 youraccount.dyndns.dk
yourdomain.dk. IN MX 20 backup-mx.post.tele.dk

The above would be correct if I were to host yourdomain.dk at home and had an account named youraccount on dyndns.dk

These lines mean that servers attempting to send mail to yourdomain.dk’s primary mail server, namely youraccount.dyndns.dk, will fail and will then attempt to deliver the mail to the backup-mx server.
There are usually a few seconds between the two attempts; in extreme cases, up to a couple of minutes.
Once the backup MX server has received the mail, it will immediately forward it to the server at dinkonto.dyndns.dk (the backup MX server is permitted to pass through the filter), provided your server is online. If it is not, the message is currently stored for a number of days, but there is no guarantee that this practice will continue.

To have the records added, you must contact your DNS provider; this will usually be the organisation where you registered your domain.