According to FAQ, you might be running into an issue with Spamcop. https://riseup.net/en/spam#spamcop
If you're describing a situation where you're sporadically not getting a few email's per day as opposed to getting nothing than you might want to check if your ISP uses Spamcop.
riseup is useless garbage. I asked and asked the people RESPONSIBLE to fix the problem, and they refuse. the problem is NOT mail bouncing back, the problem is MAIL DOES NOT GET DELIVERED AND IS NOT BOUNCED BACK. NOR IS SENT MAIL SAVED ANYMORE. I SIMPLY HAVE STOPPED USING THIS SHITTY SERVICE. SORRY I GAVE THEM MONEY.
Unfortunately, we do not have many in our collective that can speak your language. Would you mind if we used an online translation tool to help us understand one another or could you simply use English?