I don’t know what your Mail Shield sensitivity settings are, but I would suggest bumping it up to High as that should detect multiple identical emails in a time period.

It is also a little strange to see a spambot actually using your installed email client, normally they have their own very small SMTP function. Perhaps to get it through your firewall (which is ?) as that has permission.

I don’t know what your settings are or if this is even possible in Outlook, but I would see you can stop automated sending of email.