From LQWiki

DSL is short for Digital subscriber line. DSL is a broadband connection that uses the same copper wires as a regular telephone line (POTS). DSL technologies use sophisticated modulation schemes to pack data onto copper wires.

xDSL refers collectively to all types of digital subscriber lines. The two most common categories are ADSL and SDSL. Two other types of xDSL technologies are High-data-rate DSL (HDSL) and Very high DSL (VDSL).

TroubleShooting

Symptome: e-Mails are not downloaded

Reason: Wrong packet sizes

Solution: Schedule a task (see Scheduling Tasks) to set your network card's MTU to 1400 using the command

ifconfig eth0 mtu 1400

Where eth0 is your network device.