+ He could have been on holidays, or getting a child, but it is the death of his mother. Of course, we are gonna feel really bad for him. That's really good for the story.
+ The security of the system is such that on one hand a dongle is needed, but on the other hand someone can fake sending email through your account. This is apparently widely known by non-expert colleagues to just joke around, but not known by the security staff.
+ Technological "details" that do not seem to make sense such as SQL to make it sound like a true story to the uneducated.
+ The writer already assumes that we think he is lying. Hence, he comes up with the dongle story. We would already have been fine with his word that he just didn't send the resignation letter.
+ He physically shows up 6 weeks later without checking his email once (reply email). Of course the effect of the story is much stronger in that case. However, is it very likely not to check your email at least just before you show up at work 6 weeks later? [Edit: incorrect assumption, see @c8g.]
Then what would be motive of the person asking the question. It is a throwaway account.
+ A researcher who wants to see what the difference is between a fake story on Facebook versus one asked on StackOverflow? How could such an effect be properly compared?