When you have any items in your SD-Mail box that have not been displayed for at least a second, the system chimes continuously -- as a mechanism by which to pester you to look at your mail. 

Some folks did not like it this way, and have in fact resisted using SD-Mail because they found this structure so annoying. 

So now there is a new option:

Yes, just change from the default selection, and you'll get just a single chime with arrival of each new SD-Mail (or a single chime on startup if unread items are pending). 

Don't worry, visual flashing for the SD-Mail tab will persist regardless, so long as any SD-Mail items have not been displayed (which creates an assumption of having been read) for at least one second.