What is the purpose of a targeted email without any meaningful content?

Attempting to send a message to a non-existant email address will typically result in a “bounceback” message with an error code like 510 or 550 invalid address. If you try several addresses and there is no error message for only one of them, you know this one actually exists.

Someone who has a mailbox on a corporate email server also probably has access to multiple other systems or services, possibly with the same user name. The sender now has the name/handle of an account they can target on these systems.

Could have also sent a 1px image with some code to call back specific information. It’s pretty simple to do and things like this can be used for good as well (canaries). Just something to think about and check since you wouldn’t have seen it anyway.