What is a whitelist?
A whitelist (= white list) is a list of IP addresses or e-mail addresses/servers that is consulted on the mail server whether the mail being sent is valid. Mail sent from servers on the whitelist is always accepted.
This is in contrast to a blacklist. Mail coming from servers that are blacklisted is not accepted.
Does your hoster use a whitelist?
Probably yes, but check to be sure.