I have no idea of the email settings (on both systems or email programs), so I can't say for certain, but this looks like it could be avast alerting on an email. But it doesn't look exactly like the current avast email alert/branding format on an email detection so it could be fake.
To start with this if it were an incoming email with a suspicious email attachment, avast should have alerted. Either on your wife's system when sent (if scanning outbound email) or on your system when received (if not sent by her). So did either of you get an alert ?
The avast alerts:
Would change the Subject (appending as a warning to prevent the user opening the email, seems that would fail if you are opening it to do this checking.
I believe it would also remove the suspect attachment.
Since the email is supposed to have come from your wife's email account, not necessarily here computer it could have been faked too. So I wouldn't expect you to have a subject of Work in your existing emails even if this is/was legit.
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I sent myself an email with a file attached (.mht file type) that I know would trigger the suspect attachment check. However, my settings are likely to differ from yours, as I have the Mail Shield, Expert Settings, Actions set to Ask as the Primary (and Secondary action to No Action) rather than Move to Chest, etc. For the test I didn't scan outbound email.
See images.
