So far in NC, November 2020 mail ballots cast by Black voters are being rejected at *four times* the rate of those cast by white voters. https://twitter.com/WinWithJMC/status/1304747816544280577
At this point, there are two likely reasons for ballot rejection: voter error (failure to sign, etc.) and voter voluntarily canceling their ballot because they want to vote a different way. It's tough to know how much each is responsible for the discrepancy.
Either way, it's going to be a pretty big job for the parties - and let's be honest, Democrats - to follow up w/ voters who receive ballot rejection notices and help them "cure" their ballots; i.e. follow instructions properly so they count.
One thing that's clear from the NC VBM requests so far, which skew overwhelmingly white & Dem: voters of color are far more skeptical about voting by mail, perhaps with good reason. We're likely to see much greater non-white enthusiasm for early *in-person* voting.