Right click the folder, select properties and then the "when getting message for this account, always check this folder"

Thunderbird does not waste bandwidth by default on spam or trash folders being automatically updated. You have to open them for an update to initiate.

Solution works fine on imap mail, but on pop i dont have the checkbox "always check". I want to keep emails local on my computer, so if possible i prefer pop. With imap i can read emails on my 2 computers, but if i delete mail on server to make room, mail also disappears on second pc. With pop, email is kept on both computers if i delete some mail to make room. (mailbox on ziggo and hccnet are limited in size). See the screencopies.

Matt, Sorry i forgot to tell you why i want spam. I asked some firm for bill, but got nothing. When i called 4 days later they said a email was send. I looked in webinterface and mail had gone in trash. Sometimes mail gets in trash when it is not spam, so i want to check myself if all trash is realy trash. Thanks again.

your issue with spam on pop is it does not exist. Simple as that. But everyone thinks it does, so it is messy. Some providers honor the POP protocol and deliver all mail received to the "inbox" so it can be delivered. Other build a stack of cards where they filter out spam, put it in a special folder server side and simply do not deliver it to the post office for delivery. These providers are doing the wrong thing, but their customers have basically demanded they do the wrong thing because they do not want to see spam. Then there is you. stuck with whatever it is your provider has chosen to do.

Matt, Thank you, this is a very clear answer. I did not know how it worked, but now i do.