I'd been reading a bit about the absentee ballots that were rejected in my home state of Wisconsin. I was wondering, if they'd rejected upwards of 23,000 ballots in the April 2020 election, could the number be twice that in the general election? And if so, were those ballots already rejected or do they get rejected in the canvass/certification process?
This is not special to your question about Wisconsin, but in general, absentee or mail-in ballots are typically to be rejected if
a) they arrive late, i.e., after a threshold date determined from the last valid vote submission date and the typical mail delivery time plus some safety margin; this may vary with legislation, e.g., it may be a few days after polling close or on the contrary absentee/mail-in voters are required to submit a few days early.
b) they were submitted late according to the post stamp
c) there are outer envelope problems, e.g., a missing post stamp, or one can feel that there is some undesired object in the envelope, or the envelope is damaged in a way that someone might have tampered with the content
d) after opening the outer envelope, the content is problematic: missing accompanying form, or problems with the form (e.g., signer is not a registered voter or missing signature), or missing inner envelope, or ballot not put in inner envelope, or inner envelope individualized, or - cf. c) above - undesired additional objects can be felt in the inner envelope
The details may vary with legislation or form of absentee/mail-in ballot. E.g., I hear that in some legislations, a forgotten signature can sometimes be healed within a certain timeframe