Weird spam- notice appearing in my Outlook calendar without accompanying email

Over the last few months I’ve occasionally gotten an entry in my Outlook calendar that I didn’t put there. I got one today.

It’s clearly spam, something like “Your <whatever> subscription will be renewed on <date>.” There’s no more detail than that and no email or text to go with it. It’s just supposedly scheduled for some near-future date. These are things like McAfee that I don’t subscribe to and that don’t have any billing info from me.

Of course, I delete them, but geez, this seems like a really stupid type of spam. How is anyone making money on something like this? There’s no link in the entry, so it’s not like you can follow a link somewhere to buy the product.