Effectively nothing.
Ballots have already been prepared for mailing based upon the existing voter rolls and will be mailed out tomorrow.
A large percentage of those ballots will be undeliverables. An ordinary commercial mailing list "rots" at 6%-10% per year due to moves, name changes, and deaths. Once can expect a similar rate for the voter rolls. Perhaps 15% of the total mailing will turn out to be undeliverable, and thus illegal to use.
It is not clear how many illegal ballots will be harvested by individuals and even less clear whether there are organized ballot harvesting operations active.
Boulder County has a signature verification procedure and they do reject some ballots which do not match in the opinion of the examiners. I have friends whose legal ballots were rejected for this reason. Oops.
Examiners check a signature (if they do) before a ballot is opened. In theory they cannot see the actual votes when they make a determination.
It is possible to appeal a rejection - effectively this means voting in person at the County Clerk's office.
I do not know what they are doing in Denver County.
Colorado has a well-established ballot tracking system which can tell voters if their ballots have been received and accepted for counting. I plan to check it after my ballot has been placed in a county drop box. I will not be using USPS to return my ballot.
If things go sideways with the signature verification, I will traipse on down to the County Clerk office and cast a ballot in person.
My in-laws came for a short visit, a couple of years ago. I just received their application forms for mail-in ballots - from Florida. If I do receive any of those ballots, I will be shredding them.
One thing you can bet on- damned few of them will be Republicans.