Adapted from Why 'Vote-by-Mail' Elections are a Terrible Idea for Democracy by Brad Friedman

Lack of Transparency: Absence of evidence does not mean absence of fraud
As with any voting system that is not fully transparent, proving mail-in fraud can be difficult or impossible. Once we drop our ballot in the mail, we can't verify what becomes of it, and elections become a matter of faith. Additionally, should our ballots arrive in the central aggregating location untampered, they are likely to be counted by the same private, secretly programmed electronic systems that have been proven vulnerable to rigging, hacking, and undetectable error. Central counting makes fraud on a large scale easier to accomplish and harder to detect. 

Lack of Security 
Ballots are stored in hundreds of thousands of locations with no security for two to three weeks. The chain of custody lacks security as the ballots are handled by many anonymous persons throughout the process. Any unmarked contest on a ballot can be marked by someone other than the voter when the ballots are opened for counting.

Voter Intimidation 
Voting can be done as a group at churches or union halls with people looking over the voter's shoulder to make sure they vote "the right way."

Election Fraud 
There is no way to be certain that the person who signed the envelope is the person to whom the ballot was sent. Ballots can be stolen from mail boxes while the voter is at work or away from home on an errand. Other tactics include vote harvesting by persons who show up at your door to "help" you vote. The elderly and those with disabilities are particularly vulnerable.

Potential for Ballot Mishandling 
Post office or contract mailing company illegally forwards ballots; more than one ballot sent to voters; postal workers put ballots in the trash. (All of these thing have happened in Colorado)

Lack of Secret Ballot 
When election judges check in your ballot, they can see how you voted when they match the inventory number on your ballot to the inventory number next to your name on the voter rolls. The Colorado Constitution guarantees your right to a secret ballot. [ed note: as do most other state Constitutions and elections code.]

Additional Resources"Why Mail Ballots Are a Bad Idea" by Charles E. Corry, Ph.D