[foxtrottbravo]

There may be a plethora of differences but the retry delivery has nothing to do with the client you're sending your mail from.

That is definitely in the realm of the server being tasked with delivering your mail.

In traditional desktop Mail clients the Outbox is a local folder and stores Mail that hasn't been delivered to the Server tasked with sending out the mail.

The retry method mentioned here is about a Mail that has been delivered to the outgoing mailserver which in turn may have trouble reaching the destination server