It's not that the sync is failing; the entire sync mechanism in the app itself ceases to function. The sync icon turns red and going into the sync settings and clicking on "Sync now" just greys out the button. Nothing actually happens. I'm using the WebDAV method to FastMail. But again the sync isn't failing, it's not even trying. Once the app is quit and re-launched everything works again as it should (until the MacBook sleeps again).
On MBP running macOS Sierra 10.12.3 (latest) the current (and previous) release versions as well as the current 5.5.0 (75) beta all loose the ability to sync after the MacBook has been put to sleep (closing the lid) then later reawakened. The sync indicator shows red as well. Quitting and re-launching the app gets it going again.