Here's how the IRS said it happened.
Here’s how the IRS lost emails from key witness Lois Lerner - The Washington Post
Furthermore, it also stated official copies of e-mails were required to be kept locally on people's machines. However, back in 2011, Lerner's hard drive crashed. Lerner requested at the time for the information on it to be recovered, but it could not.
So the question becomes, "Is this legitimately possible?". The answer is that it IS legitimately possible, but ridiculous all the same. Let's say, for argument's sake, they legitimately do not have the e-mail records because they re-used the tape backups and her machine really did crash. It seems incredibly asinine to me for an agency like the IRS to not keep any e-mail records longer than 6 months. As a government agency, records should be kept for several years, at least 3-5 years. Where I work we don't actually archive our e-mail, but we do that intentionally (long story). But for the IRS to not have any e-mail backups longer than 6 months should be against the law itself (I don't know if it is, but it should be).
Obviously, there will be people who think it's a cover-up no matter what. There will be others who believe it's a legitimate case of lost e-mails. But to answer your question, yes, it is legitimately possible, but it doesn't make it any less bad. It just focuses the bad elsewhere.