What is a blacklist?

A blacklist (= blacklist) is a list of IP addresses or e-mail addresses/servers that send spam. This list is maintained on the mail server.
The mail coming from these servers is always considered spam and is not accepted.

This is in contrast to a whitelist. Mail coming from servers on a whitelist is accepted.

Does your hoster use a blacklist?

Probably yes, but check to be sure.