Outlook seems to be working properly but I noticed something a little odd with the sent mail, wanting to find out what causes it.

Digging through and opening emails in the Sent folder I've noticed that the majority of them open and show the Delete option in the upper left corner, like in this picture.

However, some of the emails show the Send button rather than the Delete button.  The only other thing I've noticed is that these emails often contain attachments.  At first I worried that maybe they hadn't actually sent properly but doing a few test emails it looks like they are sending properly, they just show a different option when they make their way to the Sent items folder.

Does anyone know what causes this?

(screenshot is from Outlook 2016 on a Mac, the issue occurs with Outlook 2011 14.5.1 on a Mac though)