MUA (Mail User Agent) programs support email encryption functionality for providing confidentiality of the email contents. They encrypt email contents using email encryption standards such as OpenPGP or S/MIME, mostly implemented by GnuPG, or GPG in practice. In order to understand security implication of the structures and analyze any possible vulnerabilities of MUA programs, in this paper, we investigated practical MUAs supporting e-mail encryption. As a result, we found severe vulnerabilities in a list of MUAs that allow cache side-channel attacks in virtualized desktop environments. Our analysis reveals that the root cause originates from the lack of verification and control over third-party cryptographic libraries they adopt. In order to substantiate the importance of the vulnerability we found, we delivered FLUSH+RELOAD attack on those MUA programs and demonstrated that the attack restores 92% of the RSA private keys when recipients read a single encrypted email.