So my ISP just called me again that I'm sending plenty of spam. This time, I have all flows logged, so I know for sure that it wasn't my TV (only vulnerable device with internet access) and as my switch was offline there is nothing in front of my router anymore. And I learned that all the spam was going directly to their smtp server which I never used and didn't even know they have some. All in all everything points to their cable modem. Will tell them that in response to the mail they promised to send me. Really looking forward to new at least a little bit competent ISP (alternative should be available soon).