It is worth noting at the outset that the positioning of the dialog has not been done relative to the bottom of the screen but relative to the top so the dialog permanently covers content unless the user resizes the browser height. This is not desirable, but rather than simply fixing this behavior, I'm going to propose something slightly different.
PROPOSAL:
Inspired by a website using a similar marquee for something different I threw this together using the colors of the existing dark theme. It could go anywhere on the interface. I'd recommend a pane at the top that's *sticky* when the fetch is supposedly active.
The animated 1s and 0s implies a background process is ongoing, without the user reading anything. Unfortunately I can't show a more smoother/longer animated version due to the 250Kb limit for images (my initial upload was 480Kb).
If there is a problem fetch/sending emails I can substitute 200 (Ok) with 4XX, 5XX etc and place the caution icon alongside the message.
It functions well when narrow for small devices too.
Thoughts? Suggestions? If folks like it I can add it to my list of ways to fix/improve Susimail.