Hi,
i would recommend that you just delete these and create a new backup if possible. Thats an email inside a backup thats detected. But you better ask in the mac section of the forum, can be found on the start page avast for mac.
Is this specifically a MacBook Pro and avast for Mac or also running BootCamp and that was the scan run in windows (bootcamp) ?
If it is relating to avast for Mac, then this really should be in the avast! for MAC sub-forum (more direct knowledge for Mac version of avast), as this side of the forum is for the windows version of avast.
In any case I certainly wouldn’t recommend deletion as this appears to be within a MacBook Pro email folder (likely to be an archive file, INBOX.mbox) and you could lose the emails within this inbox.mbox file. This alert is in/on 221928.emlx in the inbox folder. At least that is what I think from your message.
You don’t say what the avast detection was (e.g. malware name) ?
What scan was it that you were running ?