Learn Spanish (Spain): El jefe acaba de enviar un correo sobre el retraso. - The boss has just sent an email about the delay.

The current lesson covers por to express a cause or reason. Here, the email is about the delay (its topic), so we use sobre. If the sentence said por el retraso, it would mean the boss sent the email because of the delay (meaning the delay was the trigger for writing, but the email itself could be about a completely different topic).

What does acaba de mean here, and how does it work?

The phrase acabar de followed by an infinitive verb (like enviar) means to have just done something. So acaba de enviar means the boss has just sent it. Even though it describes a recently completed action, acabar is used in the present tense for this structure.

Doesn't correo usually refer to physical mail or the post office?

It can, but in modern everyday Spanish, correo is very frequently used as a shortened version of correo electrónico to simply mean an email.

Is retraso related to the concept of being late?

Exactly. Retraso is a noun meaning a delay. It is related to the verb retrasar (to delay) and the common phrase llegar con retraso, which is another way to say you are arriving late.