https://www.gwinnettcounty.com/web/gwinnett/Departments/Elections/AbsenteeVoting-Civilians/AbsenteeVotingMail
Top Reasons Why Your Absentee Ballot Would Not Be Counted
No signature: You did not sign or make a mark on the yellow oath envelope where indicated.
ID not provided: You did not provide ID if it was required.
Unnecessary marks: You signed or initialed your ballot.
Unauthorized envelope: You did not use the yellow oath envelope provided. If you lose your envelope or its missing, contact the Gwinnett County Voter Registrations and Elections Office for a replacement.
Unauthorized return: Someone other than yourself hand-delivered your ballot to the Gwinnett County Voter Registrations and Elections Office and you are not disabled.
Multiple voted ballots returned in one set of envelopes.
You turned your ballot into a polling location for counting. By law, voted ballots cannot be counted at a polling location.
You printed the current date and not your date of the birth on the oath.
You did not provide the address information on file on the yellow oath envelope. A business address, post office box or any other address not on file will be cause for rejection of the ballot.
Looks pretty reasonable to me. Apparently, date of birth has been a big enough of a problem for them to mention it in the instructions. And this "oath" business is a catchall referring to the envelope one returns the ballot in.
I don't know Georgia procedures, but here we examine the envelopes for a match to the voter roll and after a match is made we open the envelope and count the vote. If there's a problem with the envelope we put it aside and deal with it as an affidavit ballot. We do it that way for voter privacy, since there is no way to identify the voter on the ballot itself.
The "oath" envelope is not mailed back by itself-- it is inserted into another envelope with no voter information on it.