Mail-In Voting

There is a couple of things going on here. Let's take a closer look at the language:

"can be susceptible to fraud" is not the same as fraudulent. Even "IS susceptible to fraud" is not fraudulent. This is simply a Non Sequitur .

However, the other word, "unreliable," makes for a better argument:

Mail-in voting can be susceptible to fraud, therefore mail-in voting is fraudulent and unreliable.

This is a claim being made that because mail-in voting is susceptible to fraud, it is unreliable. It is not fallacious, but it is not a good argument either. "Unreliable" is ambiguous and means very little unless we operationalize it or even have something to compare it to, like voting in person. Voting in person is also "susceptible to fraud" (what isn't) but we generally don't consider it unreliable. Even if we do, it is an unhelpful opinion until we disambiguate (operationalize) "unreliable." For example:

P1. If a method of voting results in an error rate of more than 1%, it is unreliable.
P2. Mail-in voting results in an error rate of 1.2%.
C. Therefore, mail-in voting is unreliable.

Now we have a good foundation for a productive discussion. The interlocutors can disagree with premise one, and perhaps premise two if there is no clear data here, but they are at least in the right direction.

This is a Non Sequitur fallacy. 

The same argument can be made for in person voting as well. 

In person voting can be susceptible to fraud, therefore in person voting is unreliable.