I've check my bulf folder to see if they somehow managed to get in there. But I' can send them emails, and they reply, but I don't recieve their emails.

People are accidentally blocked quite often----and when that happens, you will be unable to receive mail from them, until you remove them from the blocked list.
On your mail page, click on "options" (upper right corner)---then click "block addresses.
Find their address(es) in the list---click on them---click "remove block".
Make sure their addresses are still in your address book.

?? Using Beta mail ??---go to mail page>>options>>mail options>>spam>>block addresses---then remove them.