Hi,
There is a fierce reason not to use the MP3 file format, that is because MP3 is a closed format for which it's necessary to pay royalties. So, it's best to use open formats such as WAV, OGG, and so on. For audio formats you can look at
Audio file format - Wikipedia, the free encyclopedia.
Depending on what is your configuration, you can get around this problem, either using another e-mail reader or browser if your e-mail account can be accessed through web interface, or even with another setting of yours.
You can also listen to and manage your voice mails through VoIP if you dial *500 from your Voxalot account.
Regards,