First, SPAM filter is way overrated. I have next to zero SPAM filter, and am doing just fine. Yes, I got lots of SPAMs, but the volume of real SPAMs is dwarfed by the volume of ads that would pass through SPAM filters anyway, so why bother.
Second, yes, open source webmail is lacking, that's why I wrote mine: https://github.com/derek-zhou/liv
Lastly, the biggest pain I have is sending email to big providers such as gmail. I have everything setup correctly, DMARC, SPF, you name it. And my server is not on any block list that I can find, and yet they put my emails in the SPAM folder from time to time. In the name of fighting SPAM, they are sabotaging the original internet experience for everyone.