Lets not start suggesting other applications just yet, considering that:
a) avast does detect it
b) because of the file type and location.
The risk of losing the .pst file and all email, this is something that should be approached with caution.
Since this appears to be an old email in the .pst file, it isn't active so doesn't present an immediate risk, but the potential risk of losing your emails is a risk.
There are no AVs that I'm aware of that are able to extract an infected email from within a .pst file without either corrupting the .pst database or deleting the .pst file. I suspect that this is true of anti-malware, so I think you get the picture of the risk of loss of all emails is higher than the risk of an infected email in a .pst file.