I don't personally use MS Outlook but it uses a method to compress multiple emails into a PST file (database).
The PST database contains multiple emails for a specific email folder (like your inbox). Currently avast can't examine the database and extract the infected email and leave the PST (database) file undamaged.
You will have to manually find the infected email and delete it from the email folder (PST file).
You will have to try to open the email folder in Outlook and in turn open each email until avast alarms then delete it and clear the deleted items folder.
Sorry I can't be more helpful than that, there may be others who use MS Outlook who could advise further.