Hey, I voted by mail every time I was deployed during elections. The voting officer would come around, find out what our hometowns were and then order absentee ballots for each of us. Never had a problem with it in the 20 years I served in the Navy. Matter of fact, I appreciated the ease of using an absentee ballot, and was happy I was able to vote.
Same here for the USAF. In Colorado, we use mail-in ballots and have a fantastic record of success. There has only been two cases of Voter Fraud until early this year and that was done by two republicans including the State Republican Chairperson that sent in his ex wifes ballot even though she moved to another state and established residency there. These were found in the After Election Probes that ALL states should have but many don't. The last fraud was finding a ballot box in a Republican Controlled County early this year that was left non picked up. It was in a Democratic area of the County. Guess what, the County Clerk (an elected position) blames everyone including the Democrats and the Elderly but refuses to accept any responsibility. Meanwhile the Republican are accepting her reasons. We, the People, don't buy her BS.
Mail in ballots are just as secure as voting in person. The chances that an illegal can vote that way is ZERO.